Fallout 3 absolutely feels like a sequel. It's been 10 fucking years, how can you possibly expect it to be a straight update to the old games. We'd all point and laugh at it for being horribly outdated.
The engine does an amazing job of conveying the huge (dangerous) wasteland and (even more dangerous) ruined cities. The VATS system does a pretty decent job of allowing some measure of turn based combat without bogging down the game and locking you into huge battles having to wait for 15 other combatants to act. It also lets you do all the stuff you used to be able to do like cripple limbs, and target weak portions of armor. The interface, specifically the amazing job they did on the new pip boy, ties everything together with the feel of the series. The SPECIAL, Perks, and Karma are all perfect for the series.
I'm not even half way through it with as an intelligent, charasmatic very good karma character, and I can't wait to play through it again being the most evil kung fu bastard I possibly can. It's a great sandbox game with a fantastic setting, and that's basically why I loved the first two.