I was consistently amazed by the scale of the game. Reading various wiki's now it seems that I did most of the quests, but I always felt like there where more out there. Between some fantastic story telling and enough variables, it really felt like it was a world to be explored, I can only guess at how long it took the guys to create all the scenery and terrain (is 200 man-years a close guess?). Lots of nice geometry tools (speedTree for foliage, morphing & colourable heads with stick on hair for people) must have eased the burden.
The shear scale of the DC wastelands was what made it for me, I come across the "you can't go any further in this direction" message once in 45 hours of gameplay (i didn't go looking for it, but you get the idea). You could explore most of the buildings in the above image, and although most weren't very involving it was enough detail to let the game world come across believably. Games like this are few and far between (LBA, and Outcast spring to mind).
It felt like I had the choice to be a good or a bad character. Being good or bad changed the way some characters reacted, but it turns out it has no effect on the main story line. Convincing story telling covered this up nicely.
It's the kind of world you can almost believe could be created by one talented (techy-ish) writer and some really clever yet-to-be-invented tools; If you take the range of art in Oblivion (a fantasy/medieval game using a similar engine) and Fallout3 and create a procedural method for creating it all then one guy could conceivably create the whole world. I think we could get the creation time of Fallout down to the length of time it takes to write a novel....?
But then we could outsource the level design, or build it up using the open-source framework as a model. Almost like a Wiki where people could create stories and subplots in their 10km2, and a player could wander a world that was expanding as fast as they could explore.
The downside of Fallout 3 (there always is one) was I had big problems with crashes in the game - I ended up playing windowed, and still getting crashes every 2 hours. Trying to use the rpg-combat system (VATS) in a busy environment (like the nice set pieces) caused the game to lock up. OS, HW and game where patched and upto date. More than a little shoddy on Brethesda's part (what about the QA guys?). Bits of the game where broken, some robots didn't appear in BigTown to let me complete a quest. But all the niggles didn't stop me playing, so it didn't really matter.
But now I might just slip back into the wastelands to see if I can find that Oasis Three Dog was talking about ;)