"Great heals."
I mean, there were fights that required me to heal, and in the early days of the new ICC heroics sometimes healing actually got a little strenuous, but none of it was anywhere near the challenge of today's heroics. That said, not once during wrath did I ever get a compliment on my healing. When I played a tank and ploughed through an instance or raid? Sure. When I out DPS'd ICC geared people in a mix of 200s and 232s? Sure.
But then I run into groups like an epic Stonecore group I had this morning. And when I say epic I do mean full of fail, as the whole run cycled through 12 people who left. But the tank we ended up getting was good, I was good, and a warlock was good. None of us had completed the place on heroic, and although a great number of DPS kept dropping (rage DC or QQ from one wipe or being totally incapable of standing out of the bad), we pushed through and persevered. Did I get any of the drops I was looking for? No, of course not. But I did become a better healer as things went along, and at the end of the day the people who stuck with me both said "great heals." It was a nice feeling -- especially when I knew I had done well AND stayed patient enough to stick through 3.5 hours of wipe after wipe after boss kill after wipe.
tl;dr: if you see a healer do well -- compliment him or her on it. They'll appreciate it.