The two characters that fell so far:
Spike - He was my favourite, or at least one of my favourite characters when he was first on the show. He started to annoy me a little post-Out Of My Mind, but I still loved him. Then S6 hit, and I hated his stupid, manipulative, whiny, creepy, mass-murdering guts. In S7 I just didn't care about him.
Buffy - I really loved her in S1-5. She annoyed me severely in S6, her wangst really drove me up the wall after a while, and at times she was so whiny and petulant that she made Dawn look mature. I still didn't hate her though. But then she took a turn for the (even) worse in S7, and became a domineering, self-centred bitca. The worst part is that it's started to colour my views of S1-5 Buffy as well, which is sad. I've started to notice that many of the qualities I disliked in S7 Buffy - overidentifying to somebody else's situation to the point of making it all about her, insisting that everyone else must follow her instructions with no argument, prioritising her creepy vampire boyfriend over everyone else and becoming blinkered - were present the whole time, they were just less noticable, and accompanied with warmth, heroism and wit, which S7 Buffy lacked.
On the other hand...
Wesley - Written purposefully for the audience to dislike in S3 of Buffy, but redeemed himself greatly in AtS. As we all know.
Angel - I never disliked him, I was just largely indifferent to him on BtVS. Then AtS brought out a whole new side to his personality and he became my second-favourite character.
Oz and Giles - I never truly appreciated how awesome these characters were until they left. Now I savour every scene with them when I rewatch.