I use to main Support particularly Sona and it was pretty much the only thing I ever played. I started playing Sona Mid and I actually ended up carrying a lot of games going like 19-4, 36-1, 23-5, stuff like that and I started to enjoy Carry role more often and currently main ADC right now. So I've had the ability to experience the best of both worlds.
: Extremely tanky, has high base damage for some reason, basically fits in and against any team comps. She counters poke I would say due to her being tanky, and CC doesn't really effect her and I think you would actually want to be CC'ed in teamfights so your Carrys couldn't be.
: He has every single thing an ADC wants a Support to have, I don't think he's better than Leona however simply due to the fact Leona can hit her skillshots almost without trying. A good Thresh main would probably still do more than a good Leona main, but I'm not 100% sure.
: No idea why he isn't played more, the buff to his damage made him able to play a lot more offensively and he's just overall a great champion. Not many fall offs, not many counters, and synergizes well with most if not all ADCs.
: Personally, I hate when anyone else plays her. They play her safe like Soraka (Who I also hate when others play) when she shouldn't be. Her ability to poke with her shield and take relatively no damage back is awesome, and putting her shield on the ADC will mean he/she auto wins trades due to it at max rank giving 50 more AD, this on top of Ardent Censer I believe allows your ADC to get 70 or 80 more AD + 15% more attack speed.
: He's overpowered, but also not. If you hit your grab, the enemy ADC is dead and there's no way around it. His damage is stupid and everything but his W has CC (actually it technically does because it Slows him after he activates it.). However if you can't land a grab or two in lane, he's pretty irrelevant unless you use your knockup and then proceed to grab them as they try to helplessly run away. He's like Thresh except a lot more damage, but much less utility and I don't know why, but everything but Ult passive scales 100% off his AP and AD I think.
: She's really underrated and personally I play her extremely offensive with her auto attacks, Q and E harrass, she's pretty nasty to fight because she can just heal her ADC back up in a few seconds and also restores a ton of health too. There's no support scarier than a Soraka with Athene's and Will of the Ancients.