League of Legends (LoL) Question: What do you think is the most important aspect for a support?

The most important skill for a support is multi-tasking. You must:
{Local}
- Manage your partner's health (through heals or damage mitigation)
- Watch the positions of the four people in your lane
- Keep track of where the minions are meeting
- Keep track of the enemies' cooldowns, especially any hard CC
- Watch the enemies' and your mana

{Map}
- Watch the position of the enemy mid and jungler
- Watch the position of your jungler
- Ward in preparation or in prevention of ganks
- Keep track of your wards' timers
- Keep track of where the enemies' wards are and their timers
- Keep track of buffs and dragon

In midgame, you'll still need to ward to protect buffs, lanes, and boss mobs, along with counterwarding ( Oracle's Elixir), and positioning yourself in teamfights to tag enemies with CC. You also might end up directing your team to make them more effective (i.e., " Tryndamere, get two wards and head to top while we defend in bottom.") and provide morale.

Being able to control the game flow by warding, making calls etc.

As a support, you don't want to sit back and wait for your team (specifically your ADC in lane) to do something, but you want to be the one setting up plays.

THE most important aspect? Depends. Here are some though:

Being able to deny as many enemy ganks as possible with good warding and map awareness.

Winning your lane by either making your carry hyper farmed/fed or significantly denying the enemy laners. There are various ways to win your lane.

Making sure the least amount possible of people on your team die when you're around. Including yourself, watch your minimap before you go put that ward in the river.

Setting up plays. In lane, time the enemy cooldowns and watch your own cooldowns (are your ADC's barrier and ult up?) and communicate with your ADC to make sure you fight at the right moment.
You can also put wards on enemy buffs or buy a lot of pinks to create pressure on the enemy and create advantageous fights for your team.

Maximize the effect of your spells. Especially the CC ones. Your damage isn't that high but your knockup/slow/stun/polymorph will always remain as useful even if you have no items. Make sure you have the right timing with your spells and that they have the biggest impact possible.

I can give you more later I guess. I'm outta ideas atm :3

Keep you and your teammates alive. essentialy your ADC. After that vision to secure your lane later vision on objectives. If you´ll manage that you can try to improve your game by tracking all that DKitten describes.