I think he means level design. Which it is, the only area that F3 is better than NV. Even the most transexual NVfags agree with that.
Level design of what, precisely? Open world, dungeons, towns? It's a mixed bag, but as far as making a believable world, Fallout 3 really is just Oblivion with guns, where as Fallout New Vegas resembles the older two titles, which have actual world building and decently written setting. Fallout 2 has a ching-chong chinatown where asians and scientologists are having a power struggle and New Reno, the most stereotypical mafia town ever made to the point where it's cartoony sometimes. The difference between making these a theme park attraction and somehow making it all fit within a game world is a matter of skill that Bethesda simply doesn't have, the rest of the locations in Fallout 2 are not just written well but provide the second deepest political intrigue you will experience in the franchise, second only to NV and it's faction focused story. TES has the same problem, in a vacuum a lot of these locations are fine, but when they are all together in one open space, few of them make sense as to how they're there or how they function and they rarely play off one another except for an odd quest that has you travel between two locations. Last TES game with decent world building was Morrowind and then Daggerfall before that.
As for towns, there is maybe 3 towns in Fallout 3 and they're all rustbuckets with barely anyone living there, meanwhile Fallout New Vegas has pretty much just Vegas/Freeside but they both feel fantastic and lived in. Then there is a whole lot of smaller towns and once again, they might not feel grand but they feel like someone lives there and that will continue with or without the player. Both games have a pretty empty open world but in this case, that's a good thing as unexplored 200 year old ruins make little sense and once again, this messes with world building big time(Fallout 4 was even worse in this regard than Fallout 3). I will relent that dungeons were better designed in Fallout 3 since there is a whole lot of them, same with combat arenas. Then again, Fallout 3 combat sucks without TTW so that's a moot point.
If there is anything that Fallout 3 actually excels at over New Vegas that would be the exploration and atmosphere the game creates. In that, Fallout 3 is unmatched and a whole lot of fun with TTW/mods to that play well with turning the game into a Stalker lite. However, when you're starting to think about how the game world works or how a community in the game survives or why the hell there is 200 year old cereal boxes laying around everywhere, that's when the cracks start showing, a post-post apocalypse experience of the West Coast will always have Bethesda beat since it's simply more believable, even when they throw wacky things into the mix as well. Bethesda made Oblivion with guns, which is what they were most qualified to do at the time, and I actually don't have a problem with that. What I do have an issue with is that by the time Fallout 4 released, that game still did not have an identity of it's own, so it became Skyrim with guns that continues the plot elements of Fallout 3 if you squint hard enough, making it Fallout 3.5 and not much more. Following that logic, I don't see TES6 being anything more than Skyrim 1.5, which is going to make people even less happy than Fallout 4 did considering that the wait is not only longer but Skyrim already has a bajillion mods which makes the new game already obsolete at launch unless there is massive improvements to the formula. Once again, Oblivion OG vs Remaster scenario.
Anyways, if there is anything to take away from what I wrote, is that Fallout 3 is a great Fallout New Vegas DLC and not much more. That comes from someone who regularly replays the vanilla game with DLC on the 360 btw so I have the pleasure of regularly being reminded of the game's flaws and the few positives it has.