Well for one, Pantheon is a Lane BULLY. He also only gets his early game, only has mid if fed, and falls off late.
The fact that he can force those champions out of lane is because he's doing his job. After that he's not as powerful whereas many of those champions are mid game.
And seriously? You complain about his ult damage but say that people should uninstall if hit by it? What's the point on complaining about his damage then? Personally I don't actively try to hit people with the ult, instead using it for positioning.
Could his kit use an update? Sure. I'd like his E active to be different, his W to be a skill shot, and have more curve in his overall power curve.
People complain that Pantheon is toxic and decide to down vote a thread asking Riot if he has updates coming up, where is the logic in that? He'll only stay as the thing you hate if you don't bring him to Riot's attention.
I'm starting to think the word "toxic" has lost its meaning for champions now. Pantheon? Toxic? Comes off as salty.