This all started back in late August, when a forum thread did, as a blue put it, "some serious sleuthing" to find out what was causing lag issues in 25-man raids. They concluded, and Blizzard's research agreed, that AoE heals were the culprits. This fix will, hopefully, correct the lag issue for 25-heroic raiders, but also hits certain spells hard.
I play a restoration shaman, but not a holy priest, a holy paladin or a mistweaver monk, so I am best placed to explain why from a shaman's perspective, but I don't doubt that similar issues will arise for the other classes affected, particularly given that the Legendary cloak procs off overhealing, which is what this specifically nerfs. Therefore, for all the classes affected, there is a risk of lowered performance in comparison to other healers thanks to these changes, particularly in 25-heroic. Unfortunately, although mistweaver monks and holy paladins are strong, holy priests and restoration shaman are not among the favored healers at this moment in time. But before we worry about that, let me explain the issue from a shamanic position.
For shaman, then, Healing Rain currently heals everything standing in it, for the same amount per tick, So, it ticks for the same numbers whether there's ten players in it, or ten players and ten Wild Imps. Non player units apparently don't count towards the DR cap divisor. The way shaman work is that every heal has a chance to proc various things, Earthliving Weapon, Ancestral Vigor, potentially Ancestral Guidance if the talent is used, and Ascendance.
All of those things, previously, had a chance to proc from overhealing just as much as they did off anything else, and the change to Healing Rain is, therefore, far more significant than just a basic nerf to the spell's output. Blizzard's team is watching this, though, and have assured shaman that they will keep an eye out for issues, and adjust tuning as they see fit. Nonetheless, it's a big nerf to one of our highest throughput spells, when we're already among the weaker healers in the current tier.
Like I mentioned, I suspect that similar issues exist for the other classes affected, but I can't pretend to know how they work, so wouldn't like to pass comment. I'm sure others will know, and clarify in the comments below, but Blizzard's current stance seems to be that reducing the lag issues in 25-man heroic raids is more important than these class-specific concerns. For now, at least, they will simply be watching out to see how the changes unfold.