Good old hypocrisy.
Not really. FNV still has sucky elements that got carried over from Fallout 3. But at least the story and game world isn't such a big ****up, which is the point here. Not that this belongs into this thread here now, though...
The point is that the "hardcore Fallout fans" already hated Fallout 3 when it was just announced by Bethesda because it wasn't 2D isometric.
Then when Obsidian made another variation of Fallout 3, suddenly it became awesome because some oldtimers from Interplay/BIS worked on it.
Actually, the point is that Fallout 3 had bad writing, bad quest design and a bad plot. New Vegas didn't. The engine was the same horribleness while the combat was marginally improved. So there are reasons to prefer New Vegas, apart from being worked on by BIS oldtimers.