My problem with Sirius was I always saw him as the 'best looking boy in school', which is what he was portrayed as, and I always forgot he spent 12 years in Azkaban.
God that made no sense, pretty much througout all the books, I'd picture what he looked like pre-Azkaban and so, thats why I was let down in PoA, because I never really pictured a Post-Azkaban Sirius.
David Thewlis did an OK Remus mind you.
Ian McKellan did an exellent Gandalf, but I don't think he'd have made a good Dumbledore, mainly because I'd always be seeing Gandalf... I loved Richard Harris' Dumbledore, but Michael Gambon did a decent job, but I liked Richards better, bless his soul.
Hagrid was pretty close, and as much as I like and respect Robbie Coltrane (go Cracker!) Hagrid still gets on my nerves.
Once again though, Alan Rickman, Dame Maggie Smith and Emma Thompson were the only ones who really lived upto my (rather unrealistic) expectations. (It's still makes me laugh, after seeing those Alan/Emma movies when they're married, screwing each other or when they were partners in Judas Kiss, and then thinking 'well, thats Snape and Trewlany together. Hehehe. My god Im so easily ammused)
Lucius wasn't what I expected, but I was, uhh, plesantly suprised, mind you
(Jason Isaacs, for his age, his hot.
esspeically with his pimp stick)