Harry Potter

Kreacher plays no role.
The battle between Harry and Voldemort takes place all over the castle instead of just in the Great Hall.
War at Hogwarts isn't as good as it could have been.
No explanation as to why Harry didn't die.
House Elves don't fight against Death Eaters.
Neville plays a greatly reduced role and isn't mentioned that he is a teacher at Hogwarts at the part where they're grown up. (Not important)

Kreacher plays no role + House Elves don't fight against Death Eaters. - I must confess I didn't miss that part in the movie at all.
The battle between Harry and Voldemort takes place all over the castle instead of just in the Great Hall. - oh, well. They had to make it somehow more \"epic\" - it's Hollywood! This is really the only part I found a bit annoying among all those not-according-to-book parts.
War at Hogwarts isn't as good as it could have been. - I found it pretty good. You have to keep in mind that the focus in the books was on destroying the horcruxes, not so much on the battle proceedings, so you can't expect to see a lot of battle scenes in the movie. I thought the scene with McGonagall preparing for battle was pretty kick-ass.
No explanation as to why Harry didn't die. - That was indeed explained, in my opinion. Perhaps your friend wasn't paying good attention or was he expecting they would literally spell it out?
Neville plays a greatly reduced role and isn't mentioned that he is a teacher at Hogwarts at the part where they're grown up. (Not important) - I think they gave Neville quite enough screen-time...