I'll have to agree with you there. OotP seemed like the first book that contained Harry Potter character assassination in it. Harry was severely dumbed down for Ron and Snape's sake. Plus McGonnagal proved herself to be a horrific head of house.

 

as for the Fidelius Charm, i see it as a warding system more than a charm like accio, and that it basically moves it to a slightly different plane. (or is it plain? english isnt my first language) allowing for entrance and exit, and connecting with the actual world.

i include portions of 5 in that, seeing as the angst harry does over Dumbladore (which is probably the inspiration for the Dumbladore/Harry slash-fans), and their probably should have been a bit darker, i read a story where Umbridge doesnt hold back, that would be an excellent addition to OOTP