It really is a fantastic game, one of the bes that I've ever played. It has all the aspects of Fallout 3, but goes deeper into each. Factions are MUCH more present, in addition to the two major factions, there are a great deal of "minor" and "petty" factions that you're free to squabble or coperate with. (I always found it bothersome that you would
always be with the BoS in 3)
There are an incredible amount of quests, most of witch have several outcomes. The setup of the world also seems to make more sense, with much less wasted space and pointless locations. Althought the layout is superior, there are noticably less "dungeons" due to the lack of a metro and there being fewer abandoned industrial buildings.
I also like how there are many more opportunities to use your skills in conversation, often allowing you to talk your way out of fights, or subvert quests all together (read: dump points into speech)
The only true flaw in the game is it's instability, the crashes and quirky bugs to dampen the experince a bit, but a handy autosave feature prevents you from having to replay too much
All said and done, it's a fantastic game if you're looking for a deep RPG with a great deal if replay value and don't mind the occasional crash or bug.