Simonson's Thor was one of the least consistent, ironically. Losing to Beta Ray Bill because of a volcano's heat one minute (even though Thor had already been in and out of the sun without problems before), demolishing the Midgard Serpent in open combat the next.
I agree, Thor has rarely ever used the full force of his powers on anyone. He tends to switch between different modes. There's thunderer mode, where he uses pretty much the weather and nothing else. There's bruiser mode (seen most often when he fights the likes of the Hulk and the Juggernaut) where he forgets about Mjolnir's versatility and hammers away at his opponents with brute force only. There's cosmic mode, where he uses Mjolnir to manipulate energy with the best of 'em. There's magic mode where he uses Mjolnir's magical abilities to defeat sorcerers and the like. Rarely do those modes ever intersect, giving us a complete Thor who's as good a fighter as his millennia's worth of experience should make him, as smart as his years as Don Blake should make him, as great a leader as basically Asgard's finest warrior/general and royal grooming should make him, as powerful and versatile as Mjolnir and his mastery over all aspects of the weather should make him, and as strong and tough as a guy who's traded blows with the Hulk and withstood nuclear blasts and everything Ego could throw at him should make him all at once. That's not even getting into the King Thor extras, which basically make him a top-tier sorcerer on top of all of that.