-- when they always run from 2v2 encounters because they don't realize you are there.
-- They die once, semi early, then ragequit.
-- They don't know where to buy the Golden Crossbow when playing Regulus
-- Sedna's who never take heal or buy priests. When you ask for a heal not knowing this, they say 'don't worry, I got healing aura'. (Oh GOOD, I'll just sit here for a minute and that'll be almost as good!)
-- They use the mouse to activate abilities.
-- They don't rebind 'ping minimap' to a key they can reach with their left hand.
Okay, maybe those last two are a bit harsh... but they are both important things you should know to do!