That list was weak and why is Duke Nukem Forever there, wtf? Not only are those Duke Nukem Never jokes really worn-out, that was really a wrong place for one. Never played any of those games and never heard of most of them...
I can't think of any really sucky sequels, probably because I've forgotten about them or just never played them, but some games that sucked as sequels but may still have been decent games:
Total Annihilation: Kingdoms
Doom3
Command & Conquer: Tiberian Sun
UT2003 (the first one after the original)
One Must Fall: Battlegrounds
everything GTA after it went 3D
and a couple of sequels I've never played but (or because) I heard they weren't really good: Carmageddon TDR2000 and Neverwinter Nights 2