I am a new member, but I have played Fallout 3 for a long time. What I would like to know is, what are significant improvements? In all honesty, a lot of the commonly stated are not very tangible in the way of recommendation and are completely opinions.
What I do know:
There is a better system for "workbench" (Aka self-making )
More Dakka (More guns and flashier ones, for those who do not know Warhammer 40k)
More clothing/equipment
What I want to know:
Are most of the new weapons really significant?
Does the ending system work better?
Has companion interaction improved?
Is the game buggier?
Is there less glamorized poor plot points (Such as Jame's useless death)?
Do the DLCs actually add a consistent amount of content (MZ, OA, and the Pitt mostly added a single quest line)?
Better ability to define your character? (Less times where there is no intelligent dialogue options, not forced to play certain ways)
Does the reputation system help define the black/white Karma system that was in Fallout 3?
Do the different join-able factions assist character diversity?

1: There are roughly 8 times as many Workbench recipes, plus a Campfire(for food and consumables) and a Reloading Bench(for making bullets). Crafting is expanded to include a large number of otherwise rare & valuable items, such as Atomic Cocktail, Nuka-Cola Victory, Antivenom, & even Stimpaks.

2: Less "flashier" guns, and more better thought through and balanced ones. Fallout 3 definitely has the more "explodey" guns, such as the ridiculously common Fat Man. However, unique variants of weapons have unique textures & usually have features which make them perfect for certain styles of play. For example, the ubiquitous 10mm Pistol is rebalanced from a starting weapon to a mid-game sidearm, generally deadly against unarmored enemies & quite lethal with a suppressor. The unique Minigun, Avenger, fires faster, deals more damage per shot, is more accurate & weighs less, but it only carries 120 bullets per magazine instead of 240. Whatever you're looking for in your specific combat style, you'll find it.

3: There is more clothing and equipment, but for the most part the existing clothing and equipment have been made more useful. Armor now provides a real level of protection, alternate ammo types allow a Guns or EW specialist to achieve new levels of lethality, poisons help Melee/Unarmed players stay relevant, and purpose-designed consumables exist for different situations, such as the Super Stimpak(which heals 2.5 times as much as the Stimpak) & a drink that causes you to take less damage from fire & energy weapons.

4: The new weapons are significant if the character invests the skill into using them well. An Assault Carbine is worse than useless in the hands of a pure Energy Weapons character, but that same carbine is an end-game weapon in the hands of a Guns specialist. There is no weapon that you "have" to use to beat certain adversaries, and no weapon that you'll find yourself progressing to as the culmination of that weapon skill.

5: The ending system gives you an individual slide for each companion, town, major faction, your karma level, your allegiance, and for a lot of the quests. There can be a total of 27 slides. I feel it closes the game better.

6: Companions now have opinions. Which they will voice, and they will proactively leave you if you trample on them. Companions also each have a quest that ends with an improvement to the companion in some fashion & with the feeling that you're helping them close off something in their lives. I feel companions are more meaningful this way. On top of this, there is a "companion wheel" which allows you to trade items, administer Stimpaks, and do everything short of firing them or talking to them without talking to them.

7: I am not qualified to comment on the game's bugginess. I can merely say that FO3 is buggier for me on my PC.

8: Almost every quest stage can be altered by the player's action(or lack thereof). You can even fail main quests, though there is always a path to ending the game. Were the James death scene redone in FO:NV, the PC would be able to hack their way into the thing with a high enough Science skill, and take on Colonel Autumn to protect their father...or to shoot their father and join the Colonel.

9: All DLCs add, at a minimum, one of each type of weapon, several armors, and a few perks. From a quest standpoint, no, not all DLCs have equal amounts of quests. Lonesome Road is very linear with a small amount of quests, Dead Money has a lot of exploring with a moderate amount of quests. Honest Hearts features multiple quests & sidequests, not all relating to the plot of the DLC at all. However, every DLC's quests fit the DLC perfectly, and offers rewards beyond quests.

10: In most every situation, your character can say whatever she would say, and get a reasonable reaction. You're not going to run into an NPC who you refuse to help and which they reply to "Ok, come back when you change your mind." There are two situations that come to mind where you can't just say whatever you want: Talking to Yes Man, and the initial meeting with the protagonists of Old World Blues.

11: Yes. Reputation controls what each faction thinks of you and whether they shoot at you, whereas karma is used on a personal level with some characters. NCR doesn't care what your karma is, they're only going to fire on you if you go around shooting their soldiers.

12: I would say yes. There's something for each character, including the ability to say "Screw them all!".

-Nukeknockout

1) Yes, most of the new weapons are quite fun and useful, far moreso than in Fallout 3. Especially when you consider the addon upgrades you can get for a lot of them. The limited and often poor-quality weapons were a weak point in 3, they are an exceptional part of NV.

2) Depends on your preference I guess, on whether you consider the ending system better or not. It gives you multiple different endings, for multiple different factions, depending on which side you choose to go with. So there is a lot more choice. But, it has no DLC's that continue the game on past those endings, like Broken Steel did for 3. I consider that a good thing, myself, some don't.

3) The game still has some minor bugs, but nothing I personally find all that bad. They've patched up the worst bugs in the past year. Most people have no real trouble with bugs anymore. There are always exceptions of course, no game works perfectly for everybody, on every kind of machine, and every platform. You'll still run into people who say it's horrible and crashes all the time and the bugs ares so awful they can hardly play it... but considering the vast majority of players have no serious problems with it anymore, you have to wonder about the health and capabilities of their specific machines and such, when they have those kinds of problems...

4) The plot and story line(s) are much more complex and varied, from 3. It is different. You'll have to play it for yourself to judge what you think, but I can't think of any totally stupid plot kinks that everybody has complained about- so no, to that one.

5) The DLC's are a matter of choice. Some ppl like all of them, others only some of them. Personally, I hated Dead Money, others think it was the best one... :shrug: No accounting for taste. But most people liked at least some of them, so yeah, I think they are worthy. If you enjoy NV, I"m sure you'll like at least some of the DLC's. They have good writing for the most part, quite a lot of content, new weapons and items, and are not just poorly-made, throwaway junk like some games make.

6) By defining your character, I'm guessing you mean in your own head, by having lots of dialogue options- yes, NV is far superior to 3 in that regard. That is a strong point of NV, and it puts 3 to shame- the devs went out of their way to make the story line(s) interesting and somewhat complex, with your actions and choices in dialogue having a lot of effects on other things that happen in the game, and how other entities react and respond to you. A lot of your choices will have consequences further along.

7) Your faction reputation system has no affect or relationship with karma at all. Karma is pretty much a non-starter in NV. It exists, and some things you do can affect your karma one way or another, but everyone agrees that the way it is implemented is useless, stupid, and has no real bearing on anything that happens in the game (you get 'bad' karma for looting stuff from the residence of an evil npc that you've killed). Might as well not even think about it at any point during the game. The most important thing about karma, is getting a PC mod that allows you to turn off karma notifications, so that stupid crap isn't so annoying. Is a non-functioning karma system a bad thing? Nah, not at all. Don't miss it, they should just dump it- it's worthless.

8) The different joinable factions greatly enhance character diversity, yes.

It would be nice if super evil NPCs have special quests and stuff for low karma characters and super good NPCs have stuff to do/options for high karma. The first 2 Fallouts had NPCs that would not even talk to a character if their karma did not match, unless the player focused on speech skill and perks...

Of course it's "better". Sequels made for same engine are ALWAYS better at least gameplay-wise.

Ooooooooooohhhhhhj you use poison on weapons?

I always though it was just a junk item to sell to traders for money...

Poison is awesome for Throwing Spears. Really makes the weapon worth it against Deathclaws. Poison isn't so much a "sell to traders" thing, you can get more money for the individual components than you can for the poison itself(what?).

To apply poison, equip a Melee/Unarmed classed weapon, then go click on the poison in the aid menu.

-Nukeknockout

....what? 1. there was only 12 Fatmans in Fallout 3, so in a way, yes, common. 2. explodey guns? what the hell are you talking about? an Explosive character barely had any type of weapons that could 'explode', we only had 4 weapons that used explosive ammo (Missile Launcher, Fatman, Miss Launcher, MIRV), and the rest was Frags, Mines, plasma grenades and mines, Pulse grenades and mines, cryo grenades and mines, Nuke Grenade and finally Bottlecap mine. Compared to any other weapon list, Explosives had the least amount in the entire game. Guns was the highest amount of weaponry, 2nd to that is energy weapons and melee weapons. So no, we didn't have 'explodey' guns in Fallout 3.

When you compare any explosive cannon to the MIRV....but yes, I see your point.

-Nukeknockout

Correct me if I'm wrong, but in Fallout New Vegas. You can kill anyone you want and there are no essential characters.

Correct. There are two characters who respawn(with an explanation) if you kill them, but you can interrupt the respawning of one of them. There is also one character who cannot be killed because he cannot be damaged because he exists within an armored booth.

-Nukeknockout

Kill literally everyone = win game?

Nah. One way or another, you need to fight the final battle. But it's possible to permanently kill literally everyone except Unnamed NPCs & Yes Man.

-Nukeknockout

How do you fight final battle if every named character is dead?

Wouldn't Yes Man win by default due to the cmfact of there's no competition?

Legate Lanius & General Lee Oliver only spawn during the battle.

-Nukeknockout

They should have made it were you can kill everyone and win by beig a huge jerk ass

Well, they made an NCR ending, a Legion ending, a Yes Man ending, and a House ending... hmmm... guess they just didn't have time left to slap together a Dork ending.

fixed.

No Moira. That's what really matters.

And no Three Dog.

that alone makes New Vegas a superior game.