What happened to the plot?

Two nights ago, I went with my brother to a late-night showing of the much-hyped 6th Harry Potter movie.  I am a fan of the books, and discovered them before they became a huge deal in the US, and stuck with the series from 6th grade through to the end of my college years, when the final book was released. 

I'll admit to not being a huge fan of the movies, and in fact avoided several of them, but decided to give "Half-Blood Prince" a shot since I'd heard so much about it.  From an entertainment perspective, it wasn't bad.  There were some stellar performances (once again I find myself loving Rowling for insisting on only British talent being used for the films, because there are some splendid, brilliant actors), some of the touches of humor that made me love the books, and the movie had some strong points from a cinematographic point of view as well. 

What it lacked, however, was a solid plot that correlated to the book.  Now, I know that the books got increasingly longer and cuts have to be made. I get that these movies have not aimed to be epic like the Lord of the Rings trilogy.  But I am baffled as to how they are going to make the final two movies work when information that is crucial and essential to finishing the battle between Harry and Voldemort has been left out of the movie entirely.   This movie focused more on the love triangle between Ron, Hermione and Lavender than it did on the impending forces of darkness that are closing in on Harry's world.  

The two highlights of the movie for me (and I am avoiding spoiling anyone, so bear with me), are the performances of Professor Slughorn and Luna Lovegood. Jim Broadbent is magnificent as Slughorn, particularly in the scene that leads up to and culminates in Hagrid's cottage.  And Evanna Lynch, how I do love you.  She is Luna Lovegood brought to life, and she is simply awesome.  I cannot wait to see more of her in the final two films.  Tom Felton fleshed out the character of Draco excellently, particularly the scene with the bird and the disappearing closet.  And, while the romance between Harry and Ginny seemed wooden and stilted, the scenes between Harry and Hermione were wonderful.

Plot criticisms aside, Alan Rickman as Snape was underutilized again, Helena Bonham Carter's Bellatrix was manic rather than terrifying, and Helen McCrory seemed horribly miscast as Narcissa Malfoy.   

All in all, a mixed bag.  I don't get why everyone else seems to be falling over themselves in praise of this movie; it was enjoyable, but it fell far short of what I had hoped, even with lowered expectations.  I just hope that "The Deathly Hallows" part 1 redeems the shortcomings of this film, but I cannot say I am optimistic.   I think I'll just wait for the remakes.