To many posters here Fallout 3 can't add to the series, only detract from it. Which is a shame because there are things it does much better than the originals.
It adds a new level of physical immersion the originals never had. It has a vastly superior exploration side (arguably by far the strongest element of Fallout 3). That adds to the series and the universe.
I find myself not captured as much by the quests (which are ok but nothing that amazes me) in Fallout 3. Combat is ok, I never really loved the combat in Fallout 1 & 2 either. Both are more functional than exciting. Sure aiming at stuff is novel for a while but eventually all combat systems get a bit repetitive. What makes Fallout 3 work is when you go into a building and spend an hour exploring it, finding out a little story that doesn't have much to do with anything (like the story I found in Springvale School). Or just exploring the wasteland, seeing something that catches your eye from afar and deciding to see what it is about.
Sure, the random buildings (dungeons) will probably get old but until now they add a tonne of atmosphere. So that is definately one of the things the game does better than the earlier games and it adds to the series.