Another couple of quick notes from me, based on a couple of games.
Losing your connection to the game counts as a leave. I'm pretty sure there's not much that can be done about this, but for someone who makes a point of never leaving a game it's a bit annoying to have that marred by a lost connection that couldn't be prevented. I've noticed there are a couple of different messages when a player leaves, but I'm not sure if they distinguish between someone leaving and someone losing connection. Perhaps a flag could be sent to the server when a user actually leaves the game/program? I dunno.
After selecting a champion for a game, when in the lobby with your team, it would be spectacular to be able to view some form of summary of your teammates' champions. I don't know the game that well yet - I don't know all the champions. Even if a mouseover just shows the tags associated with that champion, it would help a lot; if I know my team is entirely made of squishies I'll change my champion to something tanky.
When a teammember leaves the game, it would be great if their champion didn't spend the rest of the game chilling at spawn. It's a little frustrating - you KNOW you're one down, and every time you return to shop/spawn their champion is standing there, taunting you....
And finally, a team vote to forfeit would be spectacular. Some games - like where one team member leaves/disconnects early in the game - it's pretty clear early on that one team is going to win, at which point it's just a matter of time. Being able to vote for a forfeit would minimise frustration at having to play a losing game.
All that said, I would like to say that so far LoL has been incredible fun, without a lot of the frustration that I've experienced when getting rolled in DotA.