Vampire Knight -- I liked the beginning, but somewhere it stopped being about the characters and instead became about situations in which the author would like the characters to be.
Suki tte Ii na yo -- I think I liked the first chapter? Then it just became a mess of poor smut and annoying textbook shojo cliches.
Black Bird -- It made up for what it lacked in depth and originality with storytelling. Then that jumped the shark so now it's just bad.
Kimi ni Todoke -- Yeah like everybody said, it gets boring. It's slow and repetitive.