Since 5.1 hit I've been thoroughly enjoying the DPS-to-Heal style of dungeon crawling via Atonement. Well, as of next patch (5.3 the current one on the Public Test Realm) it is getting a 20% nerf. Meaning that it is no longer overpowered, which it was really. There's no getting around that.
The fact I was able to simply spam 3 spells with no regard to what order I was hitting the buttons and be able to flawlessly heal my group is a testament to the fact Atonement was overpowered.
This is less of a nerf and more of a fix really.
With a 20% nerf you may have to pay attention at certain moments but it's not broken. You may have to switch targets and use Power Word: Shield, or better yet use the strategy I described in my ALT+Z video below
In that strategy I was using Penance on the tank to create 3 stacks of Grace. That at present isn't really needed... at all. Since the nerf is only down to 80% when you apply the 30% buff of grace you are up to 110% so in order words, work grace into your rotation and tank healing will just go as it used to.
So, in spite of this nerf, for me it'll just be business as usual.