The way I saw it described today is that Fallout 2 is a better sandbox. Each zone (The Den, Vault City, NCR, etc.) is better than the first game, but there isn't any cohesiveness holding the plot together. You just sort of wander around until you stumble into the Enclave, whereas with Fallout 1 there was a clear progression.
In many ways, Bethesda were the perfect company to take up the mantle, because that's how I would describe their best games.