Kicking ass is one of the most annoying expressions I have grown up with.
To kick someone's ass seems to mean, as far as I can tell, to humiliate someone by causing them pain, or perhaps just by defeating them.
The problem with this are two:
First, of all the ways that you can hurt someone, kicking someone in the butt region is about the least effective. The butt region is well padded with muscle and fat, and has no major anatomical targets that would cause either pain or damage. In other words, if you want to hurt someone, kicking them in the butt is about the last thing you would want to do.
The second reason is related to the first. If you aren't kicking someone to cause them damage, the only reason to be doing it is to humiliate them. The idea behind kicking someone's ass is that you will be causing them some low grade pain without actually causing any permanent damage. The idea that it is possible to hit someone casually is both morally and factually incorrect. It is the idea of high school students think that bullying someone is the same as fighting. Real fights involve a lot more sharp, unpredictable and potetentially lethal force then the phrase "kicking ass" can express.
Other then that, the undertones of anal sadism in the phrase are rather distasteful to me, as well.