Thats the reason why I like it. Bad priests will just use it on cooldown or save it for "might happen" damage. Good priest will almost always find a way to combine those both. (allow me to use words good and bad, you might as well say priest1 and priest2, its just personal opinion). In same way priests can max out shadowfiend, like use it with trinket procs and maximise number of uses during bossfight, or pop it just before heroism/lust.
Difference might not be gamebreaking. Bosses die even if I raid naked with /follow when things are on farmmode. Still its all those little things combined that separates good dps and not-that-good dps.
Given interesting mechanic that aint straight dps rotation thing is good enough for me. I hope others would concider it that way as well.
What goes for "tree-defining-talent" part. I'm with the thought that VT would fit that better. Hell, shadowform is much more tree defining talent than anything. Shadowform as 31pt would be just... well shadow tree thing #1
But it would be harsh for levelers until blizz balances leveling for not having it until lvl69.
---------- Post added 2010-09-25 at 06:07 PM ----------
Allow me.
1. It does. One can't spam it mindlessly to gain all benefits. Someone might get them all but that requires planning. I'm with everything that aint mindless spamming
2. It did create "yeah!" feeling on Yogg hardmodes where other classes had mana issues thanks to long fight with current gear. Mages planning when to use evocation and cursing if it got cancelled while spriest could use it without dps downtime. YEAH won the meters and boss went down :P
3. Multidotting leveling (thou cata will change that). dot multiple targets, pop dispersion, have 100% hp/mana. Feels good man.
4. Mana cooldown on demand usable while moving, even if moving trough bad shit like in putri p3, ohshit button if mana aint needed on demand (for ex hc anub 25 there was time for 2 dispersions trought the fight, so I got plenty of timeframe to save them and use when needed)
5. It doesn't reward when fights are on farm, BUT it rewards when youre on progressmode and its all that matters pve pov.
6. Isn't this practically same question as 6. at least the answer is same.
7. What shadowtree does? In TBC it was mana mana mana (someone can link the song :P). Even in vanilla it was great manalongevity that shadow had. So all in all, shadow is spec that has plenty of mana and restores it to others as well. Dispersion fits for that "what is mana?" part of our class.