I learned that Harry is a Fraud! Finally! >:D
Cheating is awesome!
I learned that Snape kills Dumbledore. Never saw that coming.
I learned that Dumbledore knew about the horcruxes (or whatever) and was searching for them long before Harry collected the vital piece of information that told Dumbledore that Voldemort was using horcruxes.
I learned that there's a hell of a lot of drinking in this, for a kid's movie.
Heck, even the Felix Felicis potion seems to contain alcohol.
The ability to use a certain spell is dependent on whether it makes the story more interesting.
Who's snoggin' who is just as important of the threat of Voldemort.
Snape is the half-blood prince. What's a half-blood prince? Didn't learn that.
As per Seamus's comment, the appearance of one of the main characters, Slughorn, is basically pointless.
Sixteen-year-olds always do exactly what they think about doing (like how many horcruxes they will create).
I learned that the movie people must have really hated book 6, because this movie was a terrible rendition of it. They cut out important things, but just had to add the Weasley's being attacked for no reason (never mentioned again, didn't do anything to motivate the characters, no real consequences, etc.).
I learned the title of the movie was worthless, because it didn't mean anything to anyone. When the big reveal happened, it was kinda like "Yeah, and?"
Harry is worthless without somebody giving him step-by-step instructions on how to be awesome.
I learned the whole vanishing cabinet plot was a way to pad time, or Voldy just really wanted someone to break up the dishes at Hogwarts.
Happy for the Riff because it makes this movie bearable.
I learned that apparently Voldemort's assassination plan depended heavily on Dumbledore just letting them kill him, for no good reason.
I learned that wizards use their magic wands instead of a simple flashlight to poorly light up a dangerous situation, thereby making their wand unusable for anything else, or their light will go out.
I learned that disguising yourself as a perfectly clean chair in a room completely destroyed is PERFECT camaflouge.
A great wizard, who uses magic for everything, even for cleaning up, loves knitting.
I learned that one of the most powerful wizards ever, with many other powerful and very experienced wizards at his command, will choose an inexperienced sniveling kid who hasn't even finished school to kill the most powerful and experienced wizard in the world.
I learned that it is a great idea for a teacher to get teenagers to brew something call Living Death, one drop of which would apparently kill them all.
I learned that it is entirely appropriate for a creepy old male professor to make a love potion when teaching a class that only consists of attractive teenage girls.
I learned that whatever killed a giant spider is surely not a threat. Not even worth mentioning, really.
I learned that young Tom Riddle in this movie looks a LOT like Film Brain.
I learned that setting fire to Hagrid's hut was second only to killing Dumbledore in Voldemort's plans.
I learned that the villains successfully snuck into Hogwarts only to kill Dumbledore, set fire to Hagrid's hut, and then leave. And then a major part of the next two movies is their attack trying to get back INTO Hogwarts to destroy it.