My only history with playing fallout games was playing through fallout 3. To this day, I've never even SEEN a copy of Fallout 1 or 2, much less played them. But I really, really like fallout 3. So, to answer the questions...
1. Not a thing, if you ask me. It works well for a game like fallout.
2. As much as I like new age RPG's such as oblivion, fallout, and fable, my heart still lies with turn based RPG's. Maybe it's just nostalgia for a simpler time or whatever, but my favorite RPG's are the likes of Final Fantasy 3 and 7. My guess is that the creators of fallout wanted as many people to enjoy it as possible, so they gave you the option to play it as a First Person Shooter or a turn based RPG with guns. Kind of like how they gave you the option to play in first or third person.
3. As I said, fallout 3 was my first. I love the world they created, it's deep, it can be creepy, it can be funny, it feels really, well, real. I don't know what changes were made to the world or the storyline or anything, as I know absolutely nothing about 1 or 2.