I have to agree on the character part. However, I loved the missions in this game. It's the best out of all of the GTA games, and I've also been a big fan since GTA1. That said, I would love to play a GTA game that was a bit more serious.
Perhaps the next GTA game could have different characters that have stories that are separate. That way you could have different tones and themes without them compromising each other.
The missions are not great. There is a late game mission where you literally drive from Floyd's apartment to a strip club.