Harry has grown up a bit more, finds himself and his friends getting confused by love and dating, and has shed his suicidal habit of failing to confide problems to friends and allies. He's now quite clearly not a superwizard--he has a few solid talents, but is all too willing to cut corners and accept help from the mysterious "Half-Blood Prince."
And, for a change, Dumbledore is starting to give Harry some serious background on what's been going on.
The reader who knows that there's going to be a seventh and last book, and knows a bit about how dramatic tension has to evolve, will marvel at Harry's idee fixe about Malfoy, and not be terribly surprised at the form of the ending. Some mysteries begin to resolve, and the direction of the end of the series is in sight.
All in all, the book is compentently done. I will not dwell on the occasional inconsistency or stretch (the students are still unbelievably effective against the Death Eaters)--if you liked the earlier books, you'll like this one. I did.