Left click = fast attack right click = power attack. Beyond that, the distance to the targeted enemy seems to play a role in what exactly Geralt will do.
What he said but also when you have multiple enemies you are fighting, what really annoys me is I am about to finish this one guy off, but because of the retarded targetting, Geralt goes after someone else. Now if it were the case that the performance of the bad guy in terms of combat drops the closer he is to death, I don't mind that too much. But since that does not happen, because I missed killing that guy, I am off attacking someone else and then the about to be finished off guy comes in and gets in a few good slashes and drops my health a lot because Geralt for whatever reason takes FOREVER to block. It's annoying.