Support requirements?

I usually play support on ranked due to the fact that everyone else dislikes it but i was wondering what makes a good support character wise, for example you have leona and alistar who are very cc heavy then there are nunu and lulu who are more buffs and utility. but i have recently be playing fiddle support who i find is good seeing as you have a silence and a fear or gangplank who i find is more for buffs due to his e and ulti, but what makes a really good support? is there requirements?