I seem to be a support main at the moment when I play ranked. (I'd rather jungle, but I'm too willing to take support if someone wants jungle.) I've noticed a few things about ADCs
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you get the ADC who doesn't understand that CS is important. I've had Tristana's who arrive at the lane and immediately use their jump to go over the minions so they can try to 1v2 the enemy bot laners. Of course, they then complain. "This Sona is trash." because I don't follow them to their doom. They didn't try to take a single CS before launching into the 1v2 against fed champions.
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you can also get the ADC who doesn't fight. I had a Sivir who would hide behind the turret and come out only long enough to clear a wave before running back behind the turret. I was less than impressed with their CS and with the fact that I was forced to try to fight a 1v2 to keep the turret from falling.
And in both cases I was blamed for the problems.
I think a lot of it is that there aren't many true ADC mains. I know a couple of players who are trying to main ADC. They are very good about working with just about any kind of support and have a good awareness of things like wards, jungler patterns and ganks and in lane positioning. They are a joy to support because they want to work with you and will do what they can to improve things.
But many of the ADCs I've worked with have been players who wanted to play top or mid and got ADC by default because they were 3rd or 4th pick. These guys tend to lack knowledge of how to play the position, but will often blame the support.