IMO, the show started to go downhill, for me, round about season 3. I hated the fact that Minkus wasn't on there anymore (even though, it's probably due to the fact that he did the Torklesons), I hated the fact that the comedy aspect of the show seemed to have been dropped dramatically, and I hated the fact that Topanga was no longer "weird" or "unique", as I liked to call her. She was also quite individual; she lost this when season 2 started up, and it just got worse as the show went on.
I hated the way Corey's character seemed to change from the wise-guy to the, "whipped by Topanga" guy.
The worst part, though, is the fact that the show itself just became some teen-drama show. I loved season 1, and still do in fact, and am "cool" with season 2...but past there, I just couldn't enjoy it like I once did.