I agree with you about Umbridge! Voldemort was an evil bitch, but it was expected of him, and he was actually interesting as a character. Umbridge was just a horrible, horrible woman, for all the things you already said. Also, as a teacher, and one with a direct line to the person who decides Hogwarts' fate, she's a lot harder to defeat outright. She was also pretty unhinged; Voldemort, at least, had a level of gruesome predictability about him, whereas Umbridge would do anything if she felt her authority was being challenged. (I do, however, think her backstory would be interesting to explore if I could stop shaking long enough to read it. Book the Fifth took me the longest to get through because I kept getting so mad at her that I was shaking and had to put the book down until I stopped.)

I also agree about McGonagall. She is brilliant and amazing and awesome! Also terrifying, but in the 'she will do anything to protect her students, including fighting against a fully-powered Voldermort, seriously, do not fuck with her' way.

Oooh, oooh, you were a fan of Percy Jackson, weren't you? Please to be doing that one now, kaythanx.