Now, please be gentle with me. I'm a Community discussion and sometimes Skyrim member. I hear whispers of a harsh and desolate land, where members rise from the toxic swamps of the bethesda forums, to maim and devour innocents who dare call New Vegas FO:3 NV and where ancient beings preach of the decay of modern gaming and all its heinous crimes.
Anyway, I really enjoyed FO:3, having never played a FO game before, I found it a thoroughly immersive and engrossing gaming experience. From what I hear, NV is even better.
Now, bar the setting...what really sets this apart from FO:3?
Thank you forum members. Here I leave a sacrificial turtle, an offering to the Fallout gods
The most basic difference is the style of gameplay the game is designed for.
Bethesda likes to design worlds that encourage the player to give quests the finger and just run around exploring.
Obsidian on the other hand has designed
realistic locations (for example, FO3 had a Super Market used as a base by raiders, with plenty of medicine and weaponry to loot. New Vegas on the other hand will simply have an empty super market that looks like it's already been plundered, with one small goody or two, possibly in a locked safe requiring lockpicking), and the main focus is questing and interacting with the factions and politics of the area.
Don't play this game and expect to just run around exploring. There are locations to explore, sure, but what's a 30 minutes location in FO3 is a 5 minute one in Vegas. On the other hand, FO3 had maybe ~15 side quests? New Vegas must have over 90.
Another difference is the detail to those quests. Every character has a story and a lot to say. The New Vegas companions put those in Skyrim and FO3 to absolute shame. The conversations you can have with an NPC can last over 20 minutes, easily. (I did this yesterday with Caesar) And perhaps what makes it a "true RPG" in the eyes of many is that there's skill checks EVERYWHERE. Whereas in FO3, Science and Lockpicking literally meant hacking terminals and opening doors, here it also means those effect your conversation options. As an example, maybe a person has an explosive collar around their neck. With explosive skill or repair, you can disable it. With science, maybe you can at least deactivate the perimeters so the person can run away. With speech, you can lie to the person and convince them it's safe to run. With Merchantile, you can demand some sort of compensation for helping them, etc etc etc. Maybe a certain high-class gun merchant will demand a guns skill check from you before he even considers selling to you.
Basically, FO3 had these kind of skill checks very rarely. I remember it being something special when President Eden had his very own Science check. Here? They'll come up practically everywhere. Speech is a VERY powerful skill as it's often the only way to unlock special steps to a quest, OR it's the only way to skip out on a particularly difficult fight (all the bosses of the game, a BoS firing squad where ~3 of the 5 guys have Gauss rifles pointed at your skull; things like that).
Overall, something else worth mentioning is
everything you do has meaning. In Fallout 3 there was....for example, Big Town. The skill check you use to assist them effects how they defend themselves from then on out. This is cool and a nice touch, but New Vegas may take it a step further. Maybe, for example, by New Vegas standards, some of the possible solutions you can offer Big Town don't work long-term, or specific solutions will piss off certain factions, if that faction is in power in the Mojave (something that would only happen thanks to your doing). As an example and without giving away spoilers, there's a specific faction fighting with another due to a misunderstanding. Explaining the misunderstanding may bring peace to the two, sure, but depending on who ends up in charge, their peace may tick someone off, causing another fight.
Basically, think of the Mojave desert in New Vegas like a giant spider web. If you pull a thread in one corner, you may, without even realizing it, save a life or destroy one in a completely different corner. It's all VERY complicated and intertwined, which gives the game a LOT of replay value, since there's SOOOO many possible combinations of actions and outcomes.
If you're REALLY anol about playing the good guy and never doing a thing wrong, this game may frustrate you to no end. If so, I advise taking a break from trying to be the good guy, and instead doing a pure NCR playthrough, where every action you make, you ask yourself "what would benefit the NCR the most" and do it. This will free your from your personal morals for a bit and at the same time, the NCR has a LOT of quests, so playing as one of them will give you a lot of insight into House and the Legion and most other small factions.
Overall though, I'd definitely say this is the most detailed, in-depth RPG Bethesda's ever published. Yes it's better than Morrowind, in my opinion.