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Two of the most annoying things for my friends and myself is that players can decide they don't want to play anymore and DC, while reconnecting every 5-10 minutes just to prevent getting a leave, or those players who spend most of the game DC'd. This is way, when they disconnect, a timer ticks, once it hits 5 minutes they get locked out of the game and get a leave. I don't really care if they have PC problems, it just means they shouldn't be trying to play in the first place if they knew this would happen, they are ruining the enjoyment of 9 other players regardless.
The other thing is to boot players who don't ready champions in selection, so as to remove AFKers/DCers. Maybe limit this to ranked though.