Oh, the list goes on with me....I wouldn't say that I have trouble playing against them, but their 'double bluff' potential sends me into slight panic. Cards like Rhyno, with deadly damage and deadly damage reduction, and defeat cards like Ahkab where you'd rather not let your opponent get the extra pills/life but you can't afford to take damage....those are the two kinds that can knock me over slightly when it comes to strategizing.
Anything that reduces the opening play to a 50-50 guess. my favourite part is how long people will take to "think" about how to play it or how to respond to it. as if thinking about it helps... :)