disclaimer, i dont have beta but have read pretty much everything there is and have second hand info from guildies who do have beta. i also don't pve heal, at all, so my suggestions are pvp focused.
overly weak to offensive dispel
poor mana regen
My suggested fixes:
Critical heals and any Heal create a protective shield on the target, absorbing 30% of the amount healed. Lasts 12 sec.
Specificity for those who require it. All I am suggesting is that any Heal (meaning specifically the slow efficient Heal spell not all spells that heal) creates a divine aegis bubble for 30% of the amount healed via the Heal spell regardless if the Heal spell crits. Everything else stays the same.
Borrowed Time: (Rank 3)
Grants 15% spell haste for your next 2 Heal spells after casting Power Word: Shield.
Basically turn Borrowed time into a quasi tidal waves buff but only haste and only for the Heal spell and no modifiers to spell power or crit rate just a raw haste buff to the efficient heal.
Strength of Soul: (Rank 2)
When you heal a target with your Heal spell, the duration of the weakened soul debuff on the target is reduced by 2 sec. In addition if Power Word: Shield is dispelled from the target the duration of weakened soul debuff is reduced by 6s.
This isn't necessarily a nerf perse. Basically now casting 2 heals removes the strength of soul debuff and takes roughly 6 seconds to manage if you're afforded the opportunity to free cast. So shield -> debuff, cast 2 heals, debuff gone. With the change to borrowed time this would be OP as you would cut roughly 2s off this and that isn't going to work so the SoS talent needs to be adjusted down however with the haste from borrowed time to the next two Heal spells the end mechanic is roughly the same.
Rapture: (Rank 3)
When your Power Word: Shield is completely absorbed you are instantly energized with 2.5% of your total mana. This effect can only occur once every 6 sec. If your Power Word: Shield is dispelled The cost of Power Word: Shield is refunded this effect has no cooldown.
Do you dispel spam and go for the kill or just refund the priest's mana?
These changes would improve disc's throughput via Heal and divine aegis on all Heal casts would provide more fodder for dispel spam. In addition refunding the priest's mana on PW:S dispels would alleviate some of the OOM issues in PVP. They also IMO keep with the 'tanky' disc healer theme by way of hasting the Heal. I just don't forsee getting off many ~2s heal casts while tanking melee which all have interrupts now. It just isn't going to happen.
I don't think these suggestions would alter PVE much apart from a tiny bit of effective throughput gain from divine aegis procs off Heal but again I don't pve heal on any of my 3 healers so I'm certainly not the authority here. Unless of course the devs at some point have a shaman boss that spams purge on the tank at which point disc and druid healers will just /wrists and quit the game.