I know you are going to hate me because this question has been made a thousand times. But, it's been a long time that I wanted to post this.

I was writing a huge post about my experience but in the end I thought that it would be just boring to read, so..why?
After playing New Vegas, I heard that Fallout 3 was much better, at story, at atmosphere, at almost everything, so after downloading Fallout 3 via XBOX Live Bazar and expecting a super-awesome game, I started it, I was like; [censored] YEAAAAH FALLOUT 3 THIS IS GONNA BE SO AWESOME, I liked the Vault 101 part, but it was just too short for me. I usually do 2 all nighters a week, with no sleeping the next day, in a dark room with 5,1 Dolby D. Sys Headphones, so usually my gaming experiences are quite immersive. I finished it in 7 hours. SEVEN FREAKING HOURS. Why? The place, the characters, except for Dad and some others, WERE NOT INTERESTING AT ALL. The characters were not interesting enough for me to stop by and listen to their story and do their quests. In New Vegas i felt that they had more personality.
I usually love emotional gaming moments, but there was no feeling at all, usually, I feel emotion in almost every game I play, RDR, Alan Wake, GTA, Max Payne, EVEN NEW VEGAS. But Fallout 3 just felt boring to me.

New Vegas was awesome to me. It was my first Fallout game, but I knew Fallout saga before, I just loved everything, exporing Vaults, underground places, even The Brotherhood Of Steel looked better to me in NV, in a High Tecnology bunker, than in some destroyed [censored] place like the Citadel, the story made more sense to me, everything was better.
My opinion. Why do people keep saying that Fallout 3 is better?

In Fallout 3 you can't even join a faction, you can't even side with the bad guys. In New Vegas you can join anyone you want. And the endings system is SO MUCH BETTER.
I would LOVE a new Obsidian Fallout. Right now i'm playing Fallout 1 on PC, but so many compatibility problems.

PD: Enemies. In Fallout 3 I was so bored of killing yellow super-muties again and again.

Because different people will have different tastes. In terms of base game content, New Vegas is much more plot and dialogue driven, where as Fallout 3 is more or less exploration based. New Vegas can come up short in terms of desert crawling at times, but the problem with Fallout 3 is if you try to follow the logic of the quests and much of the story dialogue, it can come up short on you.

For people who more or less casually dip into Fallout, Fallout 3 may be their game because it's simple and doesn't require much emotional or thought investments, there's the clear cut good guys and the clear cut bad guys where as New Vegas requires a little more thought in its plot, there's no clear cut good guys. Some are clear cut bad guys like the Fiends and Powder Gangers, but the only real 'Good' faction is the Followers of the Apocalypse. Really it's all about what you want out of a game.

You played the game for the main quest. Definitely not one of the game's strong points. The thing that Fallout 3 beats New Vegas in is exploration. You can just head out into the remote corner of the map and find something there. Though they share the same Fallout name, Fallout 3 is a Bethesda Fallout in a sense that it shares exploratory elements of The Elder Scrolls series. New Vegas was made by some of the guys (Obsidian) who made the originals.

Notice that in New Vegas, you typically had a set path you needed to follow. Head south down the I15, east through Nipton, then north up the 95 until you reach Vegas. In 3, there is no such restriction. I'm not saying that I find any better, it's all in the mood of the player, but even though they look the same, both 3 and Vegas are very different types of RPG.

People like the exploration. There are more unique locations and neat finds in Fallout 3. I imagine some people also prefer the main questline/story in FO3, as it's much less complicated than the New Vegas questline(s).

I fire up Fallout 3 when I need that grimy, hopeless, trying to survive in a world that is (shockingly) still on the ultra-fast descent to hell. Where most people are bastards and where the good guys lose even when they win.

And I like the exploration.

Too bad it doesn't have the quests, dialogue, factions, stories, mechanics, and characters that NV has.