I actually got the interest up for San Andreas once again, you know, just to ease the itch in my fingers while waiting for V. And I enjoy it very much, frankly way more than I ever did with IV. And I see no reason to ever play IV again I'm afraid. San Andreas is a way better game than IV in every conceivable way, the best in the series so far.
But I'm sure V will be everything San Andreas is not, but especially everything IV isn't, which is basically everything.
Good-bye Liberty City
EDIT: I see you ask for concrete reasons why to hate IV. But don't take my post as a hate-post, I enjoyed GTA IV on the release date and a few weeks after but like every other GTA games from 1 to San Andreas kept me playing it for much longer than IV did.
I tried to analyze what's wrong with GTA IV but it was hard, until now. Like I mentioned earlier I started playing San Andreas again, and it has so many things to do. And the money you earn is actually for something. I don't recall being able to use your money in GTA IV for anything other than taking your undercover girlfriend out on a date or buying some cheap clothes. There's a lot of ways to spend your time and money in San Andreas.
To compare older games with IV technically is just plain unfair. There's a 4 year gap inbetween and during those 4 years huge things happened to the game-technology.
My conclusion why IV was a bad game is simply because it lacked so much to do.