There isn't really anything that I dislike about poker, but if I had to choose something that is the least among my favorite aspects of it, it would have to be that psychological aspect of it where you have to learn how to 'read' other people.
One of my least favorite things about poker; in fact, one of the things that kind of annoys me, is when people take forever to decide on their next move. I know that we need to think, but some people just take too long.
As a MTT player my least favourite thing is the long sessions I play from time to time. As I gained more experience I started to play more and more tournaments at the same time. I bought a second monitor and currently I am playing like 15-20 tables at the same time. Some of these MTTs (multi table tournaments) have fields of 5000+ people so sometimes the session may take 8-12 hours which really puts to the ultimate test both your physical and psychological skills.