Thanks Nanago, nice to see this topic here!
I think to really make a ret paladin viable in this game, Blizzard or at least the paladin community need to get together on what a ret paladin should be. We all know what we would like it to be - a supermage/priest/tank that does outrageous DPS, shrugs off almost all damage and heals the rest. *bzzzt* - not gonna happen. So let's look at philosophy - what do you want a ret paladin to be, and what are you willing to give up from the generic paladin to get it?
Some possibilities that I have gleaned from the boards:
1) DPS. Give up just about anything for a reasonable chance at topping the DPS charts given similar levels of gear to mages, rogues, hunters, shadowpriests, and warlocks.
2) Raid utility - Accept third or fourth level DPS, but have enough in the way of different buffs and/or seals to adjust the entire raid DPS a noticeable amount.
Some other possibilities might include:
a) some small auras or debuffs, something good enough to actually want a paladin in melee. For example, lower the range on Retribution aura to 5 yards, but make it actually do sizeable amounts of burst damage - say, a chance to proc and reflect all damage. Or give devotion aura an extra 2-10% resistance to all schools of magic with talent points in the ret tree. Or maybe just make all auras work as both buffs and debuffs with appropriate talents - eg, Devo aura would raise armor for all party members in range, and lower it for all enemies in range, Conc would prevent interruption in casting for party members and slow casting for enemies, and so on.
b) a true crowd control ability - either faster casting, longer effect, or wider range of mobs affected by repentance, or possibly a new form of crowd control for demons or undead equivalent to priest's shackle undead, warlocks banish undead and elementals, or druids sleep beasts. (turn undead is a poor substitute)
Just a couple talking points.