Currently HR is capped at six targets - much like AoE damage spells, the total healing done is maximized at how much you would heal six targets for, and anything beyond that reduces the healing done to every target to keep the total amount healed the same. For example, unbuffed my HR tics for around 1300. That means my total HR cap is around 7800. When I'm healing 6 or fewer people my HR heals each for 1300. If a seventh person wanders into the zone of healing, though, that instantly drops down to 1114 per tic. 10 people and I'm only getting 780 per tic, and so on downwards. HR also diminishes beyond a certain distance, but for the purposes of this discussion it isn't important.
The problem with this is twofold.
1) The 20 yard radius is too big. When I pop HR I generally want to heal a specific group of people who are taking damage. With the current 20 yard radius there is a lot of room for people to accidentally walk into my zone of healing, even when I'm not trying to heal them, and thus push me over the target cap and decrease my effective throughput. Every time some oblivious mage wanders into my healing zone he's cutting the healing my targets are supposed to be getting. Which leads into the second issue.
2) There's no way for me to tell exactly where the zone of healing for HR is. Pretty self explanatory. All of the other AoE heals that have target caps (Healing Rain, Holy Word: Sanctuary, Efflorescence) have giant glowy "stand here for the heals!" explosions of lights to direct exactly where the healing zone is. When I pop HR I get little swirlies around my toon and a whole ream of scrolling green numbers... and that's it. I have no idea if Noobie McMage is worthlessly decreasing my healing output without spending an inordinate amount of time positioning my camera to see if he also has little swirly gold lights coming out of his midriff, at which point the damage is long done.
How I would fix it:
1) Reduce the baseline HR radius to 10 yards.
2) Add a major glyph to increase the radius to 20 yards (which the appropriate distance fall-off that entails).
3) Add some sort of visual cue to where my zone of healing ends. At this point I would be happy with just a line on the ground that only I can see that says "heal ends here bro".
Given that LoD has become situational at best with the gutting of ToR and it's healing throughput, it is imperative that HR be addressed so it is at least worth the 10000 mana it costs to use, lest we once again be trapped in the tank healing only niche because we don't have the tools to operate effectively in any other role.