I'm probably too late to chime in here, but I just wanted to say, I think the way the rotation system you have going right now is intuitive and keeps things fresh. I wouldn't change a thing.. I like how it almost (but not really because they can simply buy the characters they like) forces people to try new characters out instead of sticking to their guns.
For example, this past week was a whirlwind of my opinions changing.. first I liked Rammus and I stuck with 'm getting comfortable, then the rotation came along and suddenly I was forced into trying something new and I'm glad I did , Tryndamere was a fun change and then trying out Nadile rocked everything up to the point I wasn't sure which I liked better..
perhaps there are some little slight things that you could tweak about the rotation, but I like it the way it is ^__^
since this will probably be my first and only post on these forums, (I'm not really a forum person) I also would like to take this opportunity (if anyone bothers to read it =P) to kiss some ass and say that Riot games makes me happy with many of their marketing and game-play choices. So, thanks and keep it up. ^__^