Unless you're under twelve or haven't seen (and understood) more than ten films, you're sure to find something wrong with the latest Harry Potter film. The only way to stave off disappointment would be to come in with lowered expectations, which I did.
The only thing I looked forward to were the fight scenes. The Order clashing with the Death Eaters played like a big, Star Wars battle in my head while reading the book and "Harry Potter and the Order of the Phoenix" didn't disappoint much in this regard (only that it was so short). The alternate forms of the warriors were cool, as was the resemblance to fencing, which has been the visual shorthand used since "Prisoner of Azkaban."
The rest was meh.
Unlike PoA, which made an attempt at being stand-alone, OotP just assumes you've read it and recalls the big moments for you. The Weasley twins' last hurrah before dropping out, Grawp, Kreacher, Tonks, LeStrange, DA, almost every major scene made it just because people would want to see them on screen. Their forwarding the plot in any way is simply incidental.
The problem is that the casual viewer would miss a lot of the nuances that they tried to put in. I'm not sure if anyone would notice that the prophecy was in Trelawny's voice, or that Fred and George dropped out of school, or Cho's betrayal and redemption in the DA's eyes, or even the significance of the Order (a roll call would've been nice).
That said I appreciated the small stuff: brooms sounding like motorcycles, the visual gags with Filch, the Snape flashback, the tabloid headlines; and glossed over the small things, which is how any Harry Potter film (even PoA, I guess) should be approached.