What Made Umbridge such a Hated Character?

It's pretty hard to create a character that is hated by almost the entire fandom. Let's see... Draco and Snape have his supporters. And while people weren't cheering for Voldy or Bellatrix, people still like them!

But I don't think I've ever seen a character who is hated by almost everyone like Umby here! It's almost impressive, in a sense!

So, in your opinion, what made her such a hateful character?