I'll start:
I like infinite combos. They just need to be more than 2 card combos. The more convoluted the better!
A couple of games ago I accidentally played an Unmarked Grave, which let's you tutor a non-legendary creature and put it in your graveyard, as if it was Persist, which let's you reanimate a non-legendary. Noticed it a few turns later, but kept it to myself, because it would have been a huge hassle for everyone.
Sometimes, when I read a new card, I get so excited of the possibilities, that I skip words and miss that they have additional conditions. Then I wonder why nobody else is talking about the card and why it's so cheap. Often I'll have already ordered a couple of copies and play them in my decks before somebody points out mid game that the card doesn't quite work the way I thought...