There's cutting out the boring shit, and there's outright cutting out whole characters and story arcs, even important ones, that could've improved the movies greatly.

One predominate example is the whole bullying scene between Snape and Harry's dad. What was practically its own chapter in the books was reduced to less then 30 seconds of flashbacks in the Order of the Phoenix. From the movies, we can get a glimpse why Snape hated Harry, but the flashbacks did not illustrate how bad is the bullying, and how much Snape is fed up with James and company. It also left out why Harry's mom broke up with Snape (him calling her a mudblood in a misplaced fit of rage during said bullying session), which is a rather important plot point in Snape's characterization. I felt that, have that scene was left in, the big reveal on Snape's allegiances would have been far more impactful.