It is differently a potent prayer, in all likely hood the best Cleric 6 utility power, yet there are clear opportunity costs to the power as written.
Minor action to cast, Minor Action to sustain, and only allies in or adjacent to the BIG GLOWING MAN get the healing. Move action to move the BIG GLOWING MAN 4 squares.
I have put BIG GLOWING MAN in caps to illustrate an obvious visual component coupled with a clear effect via the healing. Any intelligent foe should be able to discern the power w/in a round or two. If you do not agree with that assessment and simply have the next battle include a similar effect but for the monsters and see how quickly the players figure it out. The second monster to hit and heal will probably seal the deal. In fact if you made the power open to everyone, you would quickly see the encounter shift into king of the hill.
Given that, most monsters will attempt to keep players away from locus of the healing.
As for the healing itself, the power will heal pretty close to a 6th level healing surge (probably below for Defenders), and the Minor action to sustain, and Move Action to move 4 squares, means that unless all the cleric wants to do is solely heal....this power is going to take the place of Healing Word.
Using Healing Word with Spirit of Healing either means resources are being wasted by over healing, or the encounter is tough enough to warrant maximum healing (in which case Kudos to the power).
Likewise I find most times, people do not spend all of their Healing Surges, so again, this power either allows people to save a few more Healing Surges, which is a big woop, the person has a few more of a resource they were not going to use and which resets after an extended rest anyway....OR...the prayer really saves someones Bacon by allowing them to heal when completely out of Healing Surges or allows preservation of Surges for 1 more encounter...which again is Kudos to the power.
Tactically the wonkiest aspect would be to cast this in the back ranks and let the Ranged AOE person reap the benefit, A wizard could reap a huge reward...but again odds are this is the person w/ lowest Healing Surge value, (kudos for the power), but also behind the Defenders (cover) and thus not taking damage as much as others, and thus less effective use for the power.
Clearly given terrain, party composition, tactics, and power choice this Prayer can be exploited by the players.
On the DM side, this Prayer could very well encourage Party clumping which is easily exploited as well.