Since riot announced they want to do something with supports many people try to explain what is the problem. I realized that they are talking about the same problem but in a very complicated way. So lets make a short summary:
Other roles: lots of choices. You can focus on killing enemies, you can do split-push, you can flank enemies from rear, you can roam, etc. So those roles are fun because you can decide your own strategy and execute it.
Support rule: only duties. You have to follow the decisions of the team (even if they are bad), you need to gather information for others, you need to shield / heal / cc which is also a benefit for others.
Other roles: adapting strategy. Depending on the game state you can adapt your strategy.
Support role: in a 40 minute game you do the same thing from minute 1 to minute 40. No choices.
As a support you can try to guide the team using chat and explain the basics about the game for noobs. But that is also boring after the 50th game. You just want decent teammates, who are on the same skill level as you (people wants better matchmaking where the support MMR cannot be higher than team average MMR).
In short you sacrifice your own skills / talents for helping others. And you get nothing in return. This is where the frustration comes.
People below have an even better summary than mine:
In ANY skill level, you can get a higher rank if you play a non-support role. In a competitive game such as LoL this is a huge problem.