Honestly, the haters. I get it that people think it's stupid, that it's a waste of time and money, that it's doomed to fail (and I disagree of course, but I can understand the concept of someone who can't see what I see).
But the ferocity with which the haters come in to our house and tell us, basically, that we are fools.... it astounds me.
I think that people who spend their spare time painting Warhammer figurines are wasting their time and money too, but you don't see me dropping into Gamers Workshop every Saturday to yell at those enjoying their hobby.
And now for an answer more in the vein of what you really meant - the things that can be built on top of bitcoin. This one:
http://www.bitescrow.org/ is one recent example that really blew my mind when I realised what it could do and how it works.