I'm still fairly meh about the "Deathly Hallows" thing, and the epilogue was really cringe-worthy. Agreed about the magic system being inconsistent, too - the whole world-building sort of is, including the way the rest of the world seems to serve little function other than background for Britain. Of course, that isn't really that important in a children's series, and I doubt it bothers most readers too much.

About Snape, though, he just had to be good. All that time, Harry had been thinking that Snape was evil - because Snape was sarcastic, mean, and seemed to dislike him. If it had turned out that he was right all along and Snape *was* evil, it would have greatly undermined one of Rowling's points: that things are more complicated than just good guys and bad guys, and that sometimes people who seem like bad guys, who are really antipathetic at first and perhaps second and third sight can still be good. One could argue that Harry, Snape and Voldemort (and perhaps Dumbledore) are the three most important characters of the series, the ones upon whose moral choices everything hinges. Snape's secrets and final sacrifice were important because they illustrate that there is a way back from the dark side, and they show the kind of man Voldemort could have been if he had repented before he went too far.
And of course on top of that, if Snape had really been evil, it would have put Dumbledore's judgement into question so badly that his whole mentor role throughout the series - albeit a mentor with some dark stuff in his past as becomes apparent in the final book - is invalidated.

As far as the movies go, the acting goes from decent to very good (Helena Bonham Carter is a delight to behold as Bellatrix, and the winner of the massive auditions for Luna Lovegood, Evanna Lynch, is remarkable), but in everything from book/movie four onwards, so many things from the books are cut that I really don't like those movies too much. The first two movies clock in at over two and a half hours each, but whereas the later books got much longer than the first two, the later movies get shorter than the first two. It makes no sense, but there it is. Except of course for the movie version of book seven, as they'll make not one but two movies there - one supposes that they'll need to use a good part of the extra time they get that way to fill in the holes they created by cutting so much from earlier books.