It's been fascinating - Order of the Phoenix has caused, as it should have done, a sharp divide in the fanfiction. New information presented in new ways would do that. We now perceive Black as the "white sheep" of his family - although I want to see more about Sirius and Draco having been cousins. About Tonks and Draco being cousins (not that he'd acknowledge a halfblood.) Snape has long been perceived as a scion of a family as old and wealthy as the Malfoys - now that's Sirius' status, and his is more problematical. We've also started seeing James in a new light, and while it's not exactly flattering, it's a lot more real. And we've met Lily. *Finally*. We're also seeing Neville in a new light, and it *is* flattering. And, I haven't read many stories focussing on Ginny yet, but I'll bet she's being seen differently, too.
Plus, of course, we now have no idea what the curriculum will be like for any of the students in the next two years - except it won't be the way it is now.
And I personally am convinced Remus and Sirius were lovers, and that's going to be in future fanfic of mine. Got one fermenting now...
On the other hand - reading stories written pre OotP can be interesting, with all the various theories on how people are related to each other, and the ways Harry deals with Cedric's death, and, well. Perfect James and cowardly Neville, and the different ideas about OWLs and even the little romances. Sometimes it's okay, and sometimes it slams me against a brick wall.