Fallout 3 made the mistake of calling itself Fallout 3. it's really more of a reboot of the franchise than a continuation of it. if it was called Fallout
C or something i think some people would have found it easier to deal with.
F3 and FNV are really two very different games that look eerily similar. i made the mistake of trying to play FNV like F3 when i first started it, it's not a sandbox rpg, it looks like one, smells like one, but it's not. it's very much a semi-linear character/story based rpg that takes place in an open world space. there's very little exploration outside of quests, the world is pretty dull, and badly designed compared to F3. i keep hearing that it's the same sized map as F3 but it feels so much smaller. yeah there's more quests, but they felt very repetitive, and the choices between factions didn't amount to much, just handing in the exact same quests to different people. the atmosphere was all over the place too - centurions, elvis impersonators, cowboys, BoS with their uber-tech - it just all got really silly in the end, i guess it didn't have a coherent enough tone for me (i also fucking hate country music too, which didn't help things). say what you want about F3, it absolutely NAILS the post-apocolyptic feel of the world, the devastation, the howling wastes, it got really creepy at times. FNV's world felt like a jumbled mess that didn't really know what it was in comparison. it was just all over the shop.
having said that FNV's writing and characters were vastly superior to F3, i enjoyed the main story more too, i like the idea of being a random nobody instead of the saviour of the world (again). more variety of weapons, factions, a reptuation system (missing from Beth games since Morrowind i think), a lot of it's individual features were argurably better, but they didn't really gel for me. Beth have an uncanny knack for taking a lot of half assed features that don't always work very well, some things half broken or unbalanced, but put them all together and they somehow work, and end up being more than the sum of their parts. FNV was the opposite though, a lot of it's individual components were superior, but they just didn't fit well together. kinda like making a cake with all the best ingredients you can find, then mixing them wrong and undercooking it.
it's real subjective in the end though, F3 is a dungeon crawler, FNV is a crpg, they are both decent at what they do, take your pick.