What fixes?
Shadow priest plays like a dream right now. I tried mine out in early vashjir and simply waded through mobs thanks to self-healing and shield. Essentially infinite health, AND infinite mana with the spirit tap glyph. Not that I need the glyph really as I never popped dispersion or shadowfiend even once, I could switch out the glyph for sure and never suffer. Just use my other mana regen skills every now and then.
The new graphic on the shadow orbs is pwn by the way. Now blizzard just have to stop the buff from expiring, like they did with druid eclipses. No mastery bonus should expire, that's just bad game design.
What fixes?
Feasible in PvP, decent dmg in PvE for starters
No, "hybrid class" isn't an excuse when all the other hybrids are or have been just on par with the "pure dps classes" if not even better in both of the above.
Okay - I admit that we did have one week of PvE goodness - that was the first week of Karazhan in TBC. Then the others got gear and went flying past us even though we got the same gear too.
And in PvP we were "workable" around early BGs and Tarren Mill - Southshore, but by no means anywhere near the top classes.
Health is far from infinite especially with the newest nerfes to VE, infinite mana, yeah, that we had, but then came Cataclysm and it's far from infinite even if you use Dark Archangel, Dispersion and Shadowfiend every CD (The first zones are easily walk in the park, but it turns out quite a bit different once you get to Deepholm)