The main thing that made the relationship stuff intolerable for me was how it was presented, which plays in part to how the music was executed. The movie was literally, half romantic comedy, half action thriller. Let's look at the movie in recap:
You have about fifteen minutes of unexplained action, and you wouldn't understand any of it if you weren't a Harry Potter fan. There is no backstory given at ALL in the movie. After that, you roughly over an hour of Ron and Lavender (who was very incorrectly portrayed-- She was creepier than s**t, not really annoying) making Hermione upset, and Harry and Ginny starting to come together. Honestly, about halfway through, I forgot I was watching Harry Potter entirely. I felt like I was watching He's Just Not That Into You, or some other B-rated chick flick. The added burning of the burrow was absolutely unnecessary, except to one means that the director wanted to capitalize-- Suspense. We had Harry in a dangerous spot with Death Eaters, plus his pair for the movie, Ginny, meaning that he'd be in a position where he would have to save her. Maybe it's just the characters involved with that scene (Fenrir "Dangerous Werewolf" Greyback, Helena Bonham Carter as Bellatrix Lestrange, Harry Protagonist Potter, Ginny Save Me Weasley), or the setting (a field in the middle of nowhere), or the really shoddy acting, but it seemed like a rip straight from Twilight, except Robert Pattinson is dead. So, in short, the main events of the first hour and fifteen minutes is: Slughorn returns to Hogwarts, Slughorn introduces the students to the love potion and Felix Felicis, Ron becomes a Quidditch star and gets himself a girlfriend, Hermione is jealous of Ron and Lavender; Harry and Ginny, Ron is knocked out and says Hermione's name in his sleep which ticks Lavender off, Harry/Ginny strengthens with the burning of the Burrow. Also, the incredibly unnecessary addition of the Draco subplot you're not supposed to know about till the end. That was simply unforgivably pointless. They used four shots and six minutes to explain what Draco could have just said in one or two sentences at the climax... like he did in the book.
From the point where they start examining Slughorn's memory, the movie takes a turn for the acton. Almost none of the pairings are even so much as referenced to. Now Harry has to be Dumbledore's little busybody. He gets the memory, they delve into Voldemort's past (albeit skin deep) and get ready to find the Horcrux. Insert twenty minutes of Horcrux scene, finally a little magic, and then we're back at Hogwarts for a watered down climax with overall weak acting. By this point, it's very obvious Snape's role in the next movies, as well as his role to Dumbledore, due to various hints peppered through the movie which spoiled the overall feel of the ending.
Now you can just see, in size, which part of the movie had the greater influence-- the part that didn't matter as much! In the book, the same sort of thing was written in, but it was executed in a way so that the two naturally flowed together. It'd be like, "Plotplotplotplotplot oh, Harry making out with Ginny ploootplotplot." Why that simply couldn't be translated to film similarly, and why the two parts had to be absolutely juxtaposed, I have no idea.
[/rant]