I've been thinking about this lately(aside from Duke Nukem Forever :p). Fallout 1 is one of the best games I have ever played, but FO2 is not a good successor. It is a lot bigger, has twice as much content at the very least, but...
The humor. It is one of the most praised elements of FO2, which I cannot understand. The humor is overdone, juvenile, contrived. Way too many easter eggs, to the point that finding yet another one stops making it special. A lot of it nonsensical "lol...