Death Knights can tank in nearly any spec.
Ret Paladins can't; there's very little if any survivability in the Ret Tree other than lolbubble.
You could do some weaker Elites with Ret, on the simple strat of just burning it down before it can hurt you, but I've found it to be unreliable.
If you really wanna solo Elites, you do it as Prot with a good tank setup. It will take you longer, sure, but it is much safer. I've done many group quests, solo in Northrend as my Prot setup, though there are some quests that you just can't solo.
As far as my DK, I never attempted to get a tank setup, or an "Elite Solo" setup for her, however I did manage a couple Group2 quests with her on my own. I find that having to bite my nails waiting on Rune Tap and Death Strike cooldowns is a bit more stress than I care for; I would rather just use Healing Light, lol.
The Beauty of Prot, is that you will never run out of mana, and as long as you can defend yourself long enough to regain one holy light's worth of mana, you WILL win, regardless, unless the mob does something cheap (spell interrupts will absolutely destroy you, knockbacks are highly annoying, but managable if predictable, etc) or enrages when near death or adds respawn (you DID pull him somewhere safe, RIGHT?).
But back on the subject of Ret, Rets were never meant to be shieldless tanks like Death Knights are. They don't have any extra survivability, everything in the Ret Tree, is geared to dealing more damage.
Edit: After reading the OP again, some more thoughts:
1). Go sword and board. Seriously. The Prot Tree is useless without your shield. With BoSanc, most of your mana regeneration will come from Blocks. No shield = No Block.
2). Get the Glyph of Divine Plea, and talent that talent that makes Divine Plea refreshed by your melee attacks. use a fast tank weapon. This ensures that your Divine Plea never wears off (thus you maintain 3% damage reduction at all times, along with the simply powerful mana regen). If your self-healing falls behind, cancel Divine Plea before healing yourself and put it back up once your health is doing OK again.
3). I'm sure you know the normal prot rotation, if mana is ever an issue, drop Hammer from the rotation if there's only one mob. You could even drop Consecrate too, if needs be, if it means keeping enough mana regen to cover the cost of your heal spells.
4). Use Ret Aura, obviously. that extra armor from Dev won't mean much of anything to you, as far as damage taken.
5). Use Holy Shield every time its up. This adds to your DPS, and adds survivability -- Win/Win.
Other than that, if you still can't solo it at 80, then it just wasn't meant to be solo'd. Some quests in Northrend are particularly lousy for this, it seems Blizz decided to give most of those mobs either knockbacks, spell interrupts, or ridiculous mana drains. Most of the Icecrown Group Quests are like this, the mechanics of those quests include one of the aforementioned abilities to try and limit soloing. They know Warriors can't solo this stuff unless they are geared out the wazoo, paladins can't if they are getting spell interrupted (which locks out the ENTIRE actionbar, making us powerless, as EVERYTHING a Paladin does is considered a "Holy Spell" save Auto-Attack), knocked back constantly (can't cast if you're constantly flying through the air), or being mana-drained (no mana, you're dead).
However, they didn't come up with a death knight counter yet. Death Knights' heals are all instant (no spell interrupts or knockbacks), don't take mana (they laugh at mana drains, thought hey wouldn't laugh so much if mana drains worked on Runic Power, oh wait, yes they would. They heal with Runes, not RP).
That is why Death Knights can solo stuff Paladin's can't. However, there's a limit to how much a Death Knight can heal; he gets small heals from hitting things, he has Rune Tap, which uses a Blood Rune AND a Cooldown, and he has Death Strike which uses 1 Unholy and 1 Frost AND requires 2+ diseases up on the target to be of any use. Multiple mobs and/or mobs that do too much damage are what DKs hate, as far as soloing stuff goes...
Edited, Jul 9th 2009 9:38am by Zariamnk