Loki and Freyja are good examples, as both had to learn massive amounts of magical studies before they could do much magic. And Thor was considered more or less of an idiot. However, Mjolner had several several abilities, as well as ressurecting animals and such that he had eaten. So it could stand to say that (especially since reading several Nordic stories, I don't remember Thor having many abilities [or much mention for that matter]) prior to him obtaining Mjolner, that he had much control over anything. He was definitely the strongest, most willing to fight, and definitely considered the greatest warrior of all Aesir beings, however, like all the other gods, his limitation was really just him. He wasn't smart, so he never practiced much magic, preferring to battle due to his superior strength and power, and given the fact he was really only doing spectacular stuff only AFTER receiving Mjolner (especially given that Magni had little to no mention, or did anything spectacular until HE received Mjolner), would say that most of his powers came from his items. He was more or less just thought of as fertility over weather.