Originally Posted by
Mormolyce
INDIVIDUALLY no, they are not that hard to deal with, but one card plays ALL THREE. So you need hard removal AND AoE in one round to deal with ONE CARD. And while you're doing this, you're expending mana to deal with this one threat, failing to develop your own board, and sacrificing your AoE and removal. Leaving your opponent free to flood the board or play more high attack minions.
Dr Boom isn't some kind of magical one turn win, but it is a VERY strong play that will either swing tempo or secure a board position. Worst case scenario, your opponent has all the cards necessary to deal with it in that turn, in which case it's essentially a draw. If you have to use minions to clear him he almost always trades well.
He is pretty much the best thing you can do with 7 mana, regardless of deck, and that's why he's OP.