Well I just completed the game after about 30 hours game time.
Never played any of the other Fallout games so my overall opinion isn't going to be biased by those. The game had the chance to be so good yet turned out, as so many have found out, to be quite dis-appointing.
The game world is great, since it's been clearly built on the same engine as Oblivion a lot of the same traits are there but the feeling of a post apcoliptic world filled with people just trying to survive really is there. I identified more with the characters in this game than those in Oblivion, they felt some how more real they actually did more realistic day to day things than those of Oblivion at one point I even felt a bit guilty about turning one of the folk in to a slave, to the point were I went and rescued them later on in the game.
Combat was strange, it went from quite hard due to lack of resource and equipment at the start of the game to being dead easy with no end of ammo, weapon and armour at the end. I keep hearing the creatures were supposed to be scaled to your level but at level 20 with 100 in energy weapon and 100 small arms and the Reaper Sprint perk the only creature that even half way made me stop and think was the Deathclaw, I heard of a creature called the Beheamoth Super Mutant but the only one I ever encountered in the wilds was caged so that didn't prove much of a challenge.
So we move on to the stroy and quests. The story is uninspiring, short and comes to a rather abrupt end. To be honest once I had completed the game several times (to see the different endings) I was still of the opinion that what I had done in the world just didn't matter. The result of the final mission felt uninspiring. Of course this isn't helped by the fact that knowing what to do and where to go you could probably round the main mission up in 5 - 10 hours max. Worse still is the lack of side quests, yeah there are a few but there was opportunity for so many more, some of the side quests got shut down instantly once you made a decision or relied purely on the the player obtaining a certain skill level (blowing up or disarming the bomb in Megaton for example.)
So extending this game came down to exploring, and yes the game map is epic in size with loads of places to discover and if you feel like it you could no doubt spend many many hours exploring. Now I didn't find every location on the map but I did find a fair few and the significant problem with exploring is
a). The gains are zero, beyond finding new sources of ammo and equipment, pointless since later in the game ammo and equipment is readily at hand anyways.
b). Finding something new and interesting. Uh except there wasn't anything within the wasteland that could be classed as truly interesting, seriously how many ruined towns can you fit in or abandoned power sub stations? Their was nothing in this game that made me sit back and think oooohhh look at that I am glad I found this place because it looks amazing.
It sounds like all I have done was ***** about the game but seriously I really did enjoy this game. It pulled me in far more than Oblivion did. All it needed was
a). A more involving main quest that gives a better feeling of you doing something epic
b). More side quests, Oblivion had people throwing them at you left right and centre
c). More locals that were interesting enough so as to encourage you to explore them, seriously I found one vault went in and explored it, abandoned, overrun with super mutants. Later on I happened upon another vault, guess what; abandoned, overrun with super mutants, why would I want to explore this then?