What the hell took Katherine so long coming back to Stefan?

My best guess is that she wanted to make sure that know one would know her and try to kill her, you know make sure everyone that knew her then is dead.

Maybe it'll be a bit like in the books, where she had gone off to find her maker, Klaus. It'd explain why she's so powerful now, and doesn't even get affected by Vervain anymore. Katherine is very secretive and complicated with her reasons for doing things. After all, she says she loves Stefan, but still allowed Damon to chase after her and gave him blood to make him into a vampire. There is more to this than she is letting on, perhaps it will be revealed as the season progresses. I do hope they'll bring in Klaus though- he was badass.