From what I've seen, I'd say (in Soraka's case at least) you need top-notch map awareness to be a top mark support player. Early game, you need to keep your carry healed up, use E to top up their mana when they need it, zone your enemies back at least a bit (using Infuse + an auto if nothing else). Essentially, try to let your carry free-farm while keeping your enemies from doing the same. Once you hit six, check the other lanes to see if a Wish will be needed (e.g. an overconfident enemy committed to a towerdive, allies in a lane losing a skirmish, etc.) and use it if necessary. Ward your lane. CV as necessary (tracking a jungler, checking for a gank in an unwarded lane, checking a camp for your jungler, etc.). Midgame and beyond, you need to be where needed and keep healing/silencing targets. Never travel without your tank/carry unless special circumstances are met (helping out a close river fight, warding river/dragon/Baron), and just...support. Infuse a charging mage to push them back for a few seconds. Watch your positioning, save your Wish for a tide-changing moment, keep Astral Bless/Infuse on cooldown, etc.
Most of this isn't personal experience, but what I've seen in streams and higher level play. Lower level, supports tend to make allies a little too overconfident and such. They need to be reminded that you don't make them invincible or they go suicidal and blame you >.>.