General discussion also works as the general receptacle for complaint threads.
Maybe I am not the best person to answer this as I only play Taric and 9/10 I am the support, but communication goes a long way. Instructing them on what you would like to be done by them helps out a lot. Try to word your instructions so that your message hides your disgust for your support. If you start critiquing them they will get angry at you and will usually do the opposite of what you say just to spite you. Try something like this:
"Annie, damage is wonderful but could you buy a sight stone ASAP? It's getting near dragon time and we can get an easy kill if we have vision of it."
"Nami, don't worry about the minions I can kill them. You just worry about poking down the enemy champions. Do that and we may be able to get some kill gold."
Long winded but it works.