GTA IV disappointed me, but not at first. I enjoyed playing through the game for the first 30 or so hours, but then I realized how incredibly boring and lacking in things to do it was in comparison to it's predecessors. They made the game far, far too serious, and the only "fun" vehicle to drive was an attack helicopter which couldn't do much damage anyway. Whereas it's predecessors had monster trucks, tanks, parachutes, commercial airliners, privates jets, tanks, jetpacks... the list goes on and on. Don't forget about the fun cheat codes they had (no gravity, everything moves faster than normal, etc), while GTA IV had phenomenally standard cheat codes (Spawn vehicle, infinite ammo, change the weather, and boring stuff like that). GTA IV was a disappointment for me because it wasn't nearly as fun as it's predecessors. Luckily Saints Row has picked up where GTA left off quite nicely, and unless Rockstar get's it's act together with GTA V I'll just play Saints Row 3 instead.
Same here. San Andreas was so full of stuff to do it was insane.
And Age of Empires III was a HUGE disappointment. I loved AoE II. Great game.
Warcraft III did not live up to WC II's standards. Same with Civ III not living up to Civ II.
What is up with these 3rd games?