I believe the most important thing to be aware of before going to see this movie is that it has definitely earned it's PG-13 rating. In the pre-screening I saw last night some cautions need to be understood, more than the MPAA rating descriptor offers.
There are Snake attack's (yes, there's more than 1) from Voldemort's pet snake that could be pretty scary for those younger than 13.
Also, I saw people as young as probably 5-7 years old in attendance and those youngsters got to see more of Emma Watson/Hermione Granger than the parents probably bargained for.
For those not familiar with the book, there is an entity from Voldemort that is taunting Ron that shows him his greatest fears, which include Harry and Hermione claiming they are for each other and Ron is essentially useless. Voldemort tops it off with Ron watching this entity show Harry and Hermione from the navel up and topless, passionately kissing. With clever placement of some mist the audience is very short of being exposed to (profile view) full-on upper female nudity.
The second act of the movie consisted of a lot of waiting for the next thing to happen which, additionally, could leave the younger viewers scared at first then bored.
Despite the more mature content for younger viewers, I really enjoyed this movie, a lot! This has elements in it that I believe all Harry Potter fans have been waiting for. We get rounds of Poly Juice Potion, jinxes, broom riding, Dementors, Death Eater ambushes and "drive-by's" and wand play. My favorite part was a "shoot out" in a diner between Ron, Harry & Hermione and a couple of Death Eaters/hitmen. We also get to see Harry give Delores Umbrigde what he should have gave her 2 movies ago.
And finally, for those of you emotionally attached to the characters, take tissues, my wife was really wanting them. I even found myself pretty sad at the loss of 1 particular character, sorry, no spoilers here.