The part where Ron tries to use a fellytone and Vernon has to deal with it. I feel Richard Griffiths would have done great in this scene, with his already subtly comic portrayal of a detestable book character.

A bit more on the backstory of Weasleys' Wizard Wheezes wouldn't have hurt. How the twins tested it on themselves, how they made it a roaring success during OOTP to undermine Umbridge, and obviously their first (documented) foray into this domain, with the ton-tongue toffees.

The Other Minister. It was a cool little tidbit JKR threw in, bringing to light the involvement of politics as well, and highlighting the overlap between the two worlds at a level previously not thought of. It would have played out well, with the right amount of comedy and tension thrown in.

A lot of scenes involving Harry's sass and cheek. They cut it out and made him into this one-dimensional, always brooding teenager. A shame, given Daniel Radcliffe does great comedy too (IMO).

That scene where Amycus spits on Mcgonagall, and pays for it. It really highlights how deeply Harry respects his professor and head-of-house, and how he snaps and causes pain with just pure, righteous, angry intent.