The only issue that I see is that often there are characters who are free every other week or every three weeks or so. I know this is probably because of the need to keep some simpler starting characters available, which I completely agree with. However, it can get really frustrating when you've been waiting for a character to be on free rotation... for like 7 or 8 rotations.
For example, I've been wanting to try out Lux for a really long time, and while I've been waiting for her, Morgana has been free at least twice, maybe three times. The same goes for Sivir and a couple of others. Shouldn't one of the goals also be to keep the rotation at least roughly even? I know it's not just me that doesn't want to buy a character until I've tried them at least once.. So when I have to wait for a godawful amount of time, it seems a little silly that it isn't a little more regulated.
It seems like someone should say, "Hey, we haven't put this character free in 5 or 6 rotations, why don't we do it this week?"