You can sit down and watch an entire season of 24 without getting bored? Jesus. In my experience 24 works in sort of four-episode cycles - for three episodes a plot runs along and ends on major cliffhangers making you pull your hair out, but by the fourth episode it kind of resolves and something new starts up which allows you to get away before you get sucked in again.
The thing with the Harry Potter series is that they would make seven excellent mini-series of about ten or twelve hour-long episodes each (the same goes for Lord of the Rings, really.) This would be far too expensive for anyone to really do of course, and as it would make much less money than a single movie it's bad business too. Order of the Phoenix, compared to Goblet of Fire, is really a less eventful book. I'm sure they can fit quite a lot of what happens in the novel into the movie without having to cut entire storylines (like the 'SPEW' thing in Goblet) just by converting three separate scenes/conversations into one.
And I read that Kreacher had to be in the movie, because he still has some kind of important role in the last book. I can't remember where I saw that though so take it with a pinch of salt.