So I played around in beta 5 with a crit heal build. At level 20 I was critting every 3rd or 4th heal. It's small groups and burst heals. The first heal in warden just to top up non tank team members. I get 2 instant heals with this build at by 20 while focusing on purifier before sentinel.

I only made it to 20 in Beta so only had one instance to run but I was healing for 900 - 1200 at that point. The combat res in Sentinel looks promising too.