I saw a fun post on Reddit about supports where someone was asking what makes an unconventional support. People were tossing up all kinds of opinions such as "all that matters is utility" but I think the important issue people forget is that honestly... the support could be anything. It requires the appropriate mindset and play. But not from the support player. From the guy he's supporting.
I'm not implying the support choice is meaningless, far from it. It's just that people can't just randomly pick a support and expect the other guy to sync with that choice. I think some people just can't work with certain lane partners too different from their playstyle. The support player needs to make sure what he chooses can actually be handled by his teammate.
Hrm... let me give an example with the subject I understand the best. Myself.
When I go with a support, I tend to want certain things from them. I want a champion that has very reliable, always up hard CC (stun, root, snare, taunt) and I want someone with an aggressive playstyle. What I don't care for is heals, sustain, or even damage since I can manage all that on my own. What does this mean?
It simply means if you sent me to lane with support Ashe or even Sona, I'd be wishing they weren't even there sucking up exp as they don't do anything that I couldn't have done on my own. Even though these guys work very well as support, the choice is bad because it doesn't match the my specific playstyle. Whereas if you paired me up with Leona or Twisted Fate, boom... it's like freewin bot lane.
Then maybe there's someone who only wants his support to just sustain him and do nothing else. Then this guy wouldn't be able to work with Taric support. Or maybe there's people who want their support to do all the harassing for them while they just sit back and farm. So he wants Zilean or Nidalee and would be pretty pissed with Soraka.
That's my theory on what actually determines the viability of a support anyway. That contrary to popular belief, the best way to pick a support is based on what your partner can handle... not you. At least, that's what it feels like from my experience.