I read books one through five when they were all readily available, with many months left before six came out. Because I could read those first five one right after the other, they felt more cohesive, and that sense of the books grew in those last remaining months. Book six felt like a slightly different reading experience, as did seven.
But bleeding-reeming hell! The lag between readings is the only reason for that! I get that new canon can feel like fanfiction if it's been a while since the last bit of canon material got out, but all seven books rang true to her style for me all the same.
The other reason that the last two books "felt like fanfiction" is that there had been enough time and enough obsessive theorizing that SOMEONE HAD WRITTEN AT LEAST ONE FIC IN WHICH AT LEAST EVERY MAJOR PLOT POINT POSSIBLE HAD APPEARED.
Heck, one of the first conversations I had in online fandom (this was two years pre-HBP) had to do with Voldemort hiding a piece of his soul in Harry's scar. Take that, JKR!
Snape loving Lily? Harry pining for Ginny? Dumbledore killed by Draco/Snape-who-really-did-it-because-DD-was-going-to-die-anyway? Read 'em all.
Except the Hallows. I don't think anyone saw those coming. ;-)
Could play into it. The only way I can really relate to that claim is through getting used to any new text about Harry and Sirius and so on coming from fanfic, so it's jarring for a moment to read about them in new canon.
That moment goes by pretty quickly. 1-5 are my nostalgia books, but I have no real complaints about HBP and DH.
Man, I groaned at first over Snape/Lily, but only because I associated it with its fans. Looking back, it was sorta hinted at in his worst memory and all that, so it shouldn't have been a surprise. It fits. I just like to think that part of his obsessing over her all that time was due to guilt over having caused her death, and not soley him never being able to let go of a high school crush.