Honestly champs with alot of sustain like healers is really annoying, part of the mode is running out of mana and health. Also, Blitzcrank's arm pull is more frustrating than any CC because it pretty much is instadeath if he grabs you and its 5v5 lane. You are separated and dead. Also, teams often get a ton of CC and it makes team fight impossible. Anivia is another champ that is super annoying because of the ult. You work so hard to kill her and then she pops the ult, you dont have time or safety to kill her because it is an aggressive 5v5 match with no chance to kill her again. And then we go back to the sustain. It gives her tons of time between deaths and makes her super hard to kill. Also, Ezreal is annoying because the lane is about the width of 2 of his ults so really, its super hard to avoid and makes it hard to fight and live. Of course fighting against stupid OP champs sucks because then you die a lot and lane gets destroyed. But really, in ARAM, it sucks when you get an unbalanced team. I fought a team with 4 AP champs and Sivir. Thats gotta suck. It should randomize no more than 2 champs per role so that you have different and interesting combos but not get screwed because of lack of tank or too much tank and no damage. All match ups can be overcome with good teamwork and strategy though.
Oh and i did forget to say Master Yi. I hate him so much. He is way too OP whether he's AP or AD. AP makes the dash killer and stupid hard to deal with..and AD Yi puts out so much damage plus the Attack Speed everyone gets makes him impossible to kill or survive. PLUS hes got the heal sustain which makes him impossible to kill because he just keeps coming back.