I loved Fallout 3 when it came out. But even then, I knew it was flawed. Whatever technical criticisms I could say about it, I think my biggest issues is pacing in certain sections (in terms of main missions and progressing the map), as well as certain sections feeling tedious.
As others have said, I just found Vegas did a lot more right. At the same time, I can't hate on Fallout 3. Yeah it's dated, and it's flaws are a lot easier to see after time + the release of New Vegas. But the game also had really awesome side missions, and just things about the world I loved. No doubt playing it at release, means I can think of it a lot more fondly. But I still think there are things about it that still hold up today, regardless.
I wasn't a big fan of Skyrim. I thought all the side missions, towns, characters, and general writing, was much better in Morrowind, Oblivion and parts of Fallout. So my biggest fear is that they focus too much on making Fallout 4 a technically incredible/vast world, but drop the ball on filling that world with funny/creative/interesting characters/quests and locations. Skyrim felt vast to me, but also felt hollow. The world felt empty in many locations, and I generally didn't like the writing as much as their previous games (although Skyrims main quest was actually decent).