I don't really have any issue with bitcoin itself but my least fav thing is the lack of real world places to spend it. I suppose the confirmations could be a little quicker but it's not really an issue to be honest.
The least favourite thing about bitcoin to me is that it takes to long time to get the first confirmation. We need to increase the block size.
But yesterday i noticed that now transactions do not take too much longer time, as yesterday a have transacted more than 1 bitcoin and te transactions got its first confirmation within a few minutes.