Just that Fallout 3 was a cohesive world, that had soul. Yes, it had some flaws, and yes, New Vegas iterated on it and added many more features, however, the world building, and that elusive "soul" were gone.
Just like MW3's campaign did not have the same soul as MW1-2. While those games were certainly not without flaws, they had a certain feel, and style to them.
Kotor I vs Kotor II, or Uncharted ND vs Golden Abyss.
I am not saying that New Vegas is some terrible game. Hell, I would have rated it higher than its metacritic.
That said, everyone likes to shit on Bethesda while praising Obsidian. While the reality is, the game engine, gameplay systems, many assets, art direction, etc was already in place by the time Obsidian was contracted to make New Vegas. I would like to see Obsidian make a game like Fallout 4, without using another game as a base. Build it from ground up.
People just don't understand the iterative process, and attribute many of Obsidian's accomplishments to stuff that Bethesda would have done in Fallout 4 anyway.
Now what I am not saying is that the writing in F03 is amazing or that the Writing in NV is terrible. And I have already acknowledged multiple strengths of FO NV. But still you have people replying with .gifs, and calling me a joke because of my opinion, which I have tried to actually express and not just drive-by post.