Too many free-roaming features (parachutes?? planes!?!?) taken out, leaving it less satisfying than San Andreas. After the story ends, there is far less to do in IV than there was in San Andreas. I probably spent ~90-100 hours in San Andreas, maybe 50 in IV in the solo campaign. The multi-player makes sure that I'll keep the disc around for future entertainment, but until the Rockstar PS3 servers are capable of saving stats (I've played probably 10 hours, yet every time I restart, so do my stats), I can't see playing online as much as a COD4.
Those items, however are just the negatives. Aside from the above, it is still a solid game that will take plenty of time to play, and it is still GTA.