Fallout 3 was a good game for the time before New Vegas, but then New Vegas showed up and it proved just how much of a shallow experience Fallout 3 really was.
In terms of lore, Fallout 3 didn't just give the older games a disservice, it absolutely destroyed it. To a point where Obsidian had to try to amend it as best as they could with various additions to the lore to try and fit Fallout 3 with the other games.
In Fallout 3, the roleplaying aspect is totally broken. You could walk all over deathclaws by the time you was level 20 (Dart Gun - Deathclaw legs - Instant Win!), and some of the perks were overpowered it was ridiculous (yes, Grim Reaper Sprint, i'm talking to you). This isn't even touching on the speech system, which was completely and utterly ridiculous too. You probably only need about 10 points in the speech skill, then you don't have to worry about it at all. Then if a convo didn't go your way, you could just reload a save, making it a truly flawed gameplay mechanic. In New Vegas however, speech is the most important skill in the game, which makes for far better roleplaying.
I also apperciate Hardcore mode in New Vegas, which makes for a even more immersive experience. While getting food, and water can be a pain sometimes, it's far better than drink feeding stimpacks like you do in Fallout 3. Not to mention Stimpacks are like sweets in Fallout 3, so it's hard to even die.
While Fallout 3 had some really cool locations, how much of those were the same copy and paste dungeon, or metro station. It was like Oblivion all over again. New Vegas on the other hand is a case of quality over quanity. The Vaults were a lot better, and had better backstories than Fallout 3, and the lack of copy and paste made you want to explore the mojave, and see every nook and cranny. Fallout 3 bored me to tears by the time i found out that I was going through practically the same places.
Fallout 3 was devoided of typical Fallout charm as well. With only very few memorable characters, such as Three Dog for example. New Vegas makes you feel more attached to these characters, and the post-apocalyptic world they inhabit.
The DLC's for New Vegas seem more well rounded too, and they actually make sense in the world. They also give different varieties of play as well, with Honest Hearts being a focus on the Survival skill, while Old World Blues focuses on the Science skill. The DLC's for Fallout 3 were a mixed bag at best, with some being good, while some were terrible, and i mean terrible. Seriously, if i saw the bright sparks who thought up both Operation Anchorage, and Mothership Zeta, i would give each and every one a swift kick in the bollocks.
I just can't play Fallout 3 now, I see too many annoying flaws that were addressed in New Vegas. So why play a deeply flawed game, when you can play a similar game which has all these issues addressed.
It's not perfect, but Fallout New Vegas proves to be a worthy succesor the the orginal games, and it's one of the very few things that makes me glad that Van Buren wasn't released, and Fallout 3 was instead. Otherwise New Vegas wouldn't be the game we all know today.