in my opinion i think that as the books get longer they get .. well not worse... but they start to drag and i don't think that either GoF or OotP were paticularly strong, they had good bits, like the ending of GoF was great and the 1st task but inbetween it was a bit slow and the same goes for OotP, there was just a quality in them that was lost or something. but i think the perfect length was PoA, it was long enough and not too short, perfect, it didn't drag at any time and it was by far the best.
also just re-read OotP cos i hadn't read it in like a year (too painful, sirius come back
) and it definitly makes you uncomfortable to read but not in a good way, tragedy should leave you with catharsis and i needed that when sirius died and i got...nothing. had to emnotionally cleanse in my head, but the book should do that! and i stand by my probably unpopular view that harry was badly written in that book, very flat, 2-D characterisation of his anger that to me felt very clunky and to me off-putting. i can see the intended effect but.. nah didn't work for me, but hey differrent people like different things.