(Warning: spoilers) more powerful in the book

The revelation that Snape is the Half Blood Prince:

In the book, more focus was put into the mystery of the identity of the Half Blood Prince, and what the title entailed. The movie did this clumsily, and, in the end, made you uncaring of who it was. It felt more like an afterthought than an actual revelation.


Note: It also would have been great to see Snape teach Defense Against the Dark arts. This would have given Harry the opporunity to say my favorite line of his:
"There's no need to call me 'Sir', Professor."
That line was a gem, and it wasn't in the movie, my disappointment runs cold.

The Cave:

This chapter in the book was very powerful. It made you feel for Dumbledore, but even more for Harry, who was torn between the orders that Dumbledore had given him, and his own morality. In the movie it felt rushed, and the emotions felt stilted and slightly false.

The Unbreakable Vow:

Another scene where the emotion was much more believable in the book. You really felt the love that Narcissa Malfoy has for her husband and son. This is the first time you see how far she would go to protect her family. In the movie, it was much more ritualistic and rehearsed.

Dumbledore meeting Tom Riddle:

This scene really shows the personality of the boy who became Voldemort, and the first signs that he is a sociopath.

The relationship between Harry and Ginny:

They put two of the romantic plots in this movie, but they killed the most important one. I would have liked to see Harry and Ginny's relationship evolve the way it did in the book. Instead, we get a disappointing kiss and an offhand remark. There was no room for it to grow because they wasted time on the Ron/Lavender/Hermione love triangle that went nowhere.

Rufus Scrimgeour:

We should have met the news minister of magic in this movie. The Dynamic between Harry and Scrimgeour was amazing in the book, and it was a chance to show Harry's loyalty to Dumbledore.

(No picture) These scenes were, unfortunately, omitted.

The evolution of Tom Riddle:

We missed the moments that made Voldemort who he was. And, in the end, we have no more understanding of him than we did before. Plus, we miss out on the most important information in the series (what some of the horcruxes are). I don't know about you, but I'm not sure the next two movies will be very interesting if they are just about Harry flailing blindy trying to find the pieces of his mortal enemy's soul.