As far as looks, I don't care too much because I am not paying attention to the guy. If he's got something disgusting about his physical appearance, then yeah it can be distracting. Usually his main job is just to highlight the true artwork (female pornstar), maintain the mood and the tempo, be a good prop for her to work off of, and not be too much of a distraction.
So for male pornstars it's more about what they say (or more importantly don't say). For the most part, I like ones that are funny before and after the action and pretty much shut up during....
Van Damage, Tony T., Frank Wank and some others come to mind...