Comment below rating threshold, click here to show it.
First bit of feedback here - I've only been playing the last couple of days, so bear with me...
When you view the Summoner information screen (for example), it is insanely easy to quit the game when trying to leave that screen. I've only done it the once so far, but my automatic action when I want to leave a screen is to click the x in the top-right corner.
As things go it's not a huge concern - the worst of it is a couple of seconds to reload the game. I don't suggest the layout of the screen be changed, not at all, but a confirmation window could be a good idea.