GoF and OotP both needed a lot of fixing... but since Browny did GoF I'll do OotP...
First off, Harry would yell at Ron and Hermione, like he's supposed to. And Dumbledore. And the memory scene would be a lot longer... with Lily! And Snape calling her Mudblood! I swear, if they cut out The Prince's Tale in DH... *trails off muttering darkly*
And I'd have to include the fact that Ginny has a boyfriend... and when she deals with Harry's temper TWICE... have you noticed how good she is at that?
And... hmm... I'd include Ron & Hermione being prefects, and Ron becoming Keeper...
And all the extra stuff from the Department of Mysteries that they cut out... and no crying from Harry... he didn't cry...
Good grief, they messed it up badly.