Which is my current build. I tank absolutly fine in Karazhan. I normally do not tank bosses, but for group pulls and as a OT I'm a dandy. I think I could tank every boss, with the exception of the Prince and Nightbane. Which I haven't tried.
The reason this build is fantastic is because you're still getting the best of the healing world. With the exception of 3 pts in Light's Grace to reduce HOly light 100% of the time cast by .5 secs. Which helps as a main healer. However I can still successfully main heal heroics and anything else I'm up against. But toe-to-toe against a full holy spec pally, they're going to beat me in healing.
Onwards to tanking.
with 20pts in Prot. and good tanking gear (not to mention knowing how to tank as a paladin). You'll be fine. The key here is the following.
When a paladin tanks there are two main areas they need to watch for.
1. High front load HOLY DAMAGE, followed with steady holy damage until the encounter is over. Paladins hold aggro by doing HOly damage, it's how they build threat. Which is why "righteous fury" is important. The new version is even better because not only does it increase holy damage threat, it also decreases damage done to you when you have it active.
2. The other area is mitagating Damage. A paladin tank needs +def and +stam gear to absorb hits.
The fun thing about a paladin tank is that when the going gets tough. They heal themselves. This is why I have the points in Concentration aura. Almost impossible to stop the heals from yourself to yourself +you don't get silenced or interuppted as often, thusly you can still use your holy attacks. I usually throw another Paladin in my party with Protect aura up.
With the passive mana regen from getting heals a paladin tank almost always has mana. Since healing yourself generates threat alot of times I will front load Holy damage,
this is how it works, Pop Divine favor, run to mob, Judge Crusader, hit divine illumination, hit avenging wrath, hit hero's charm, seal of might, hit exorcism, holy shock, consecration (when mob is in place), judge might, and rinse and repeat holy shock, exorcism, and judging might, rejudge crusader whenever it's needed. That is how you front load holy damage. When you need to start healing yourself with flash of light or holy light. I usually spam Flash of light once i have good aggro just to help the healers out.
That of course works great on undead. for regular mobs just don't use exorcism.
For large undead group pulls, I pull the mobs to me using LOS, stop cast consecration, immediatly cast holy wrath, cast consecration whenever it's up. If you have imp. righteous fury, concentration aura, and good tanking gear, and good healers then tanking AOE undead groups is a cakewalk in this spec.
For heroic instances you can tank fairly well in this spec, but honestly if you don't have an awesome Healer, awesome tanking gear, and DPS who knows how to hold off on attacking... Don't even attempt it.
I'd say a full tanking Paladin could handle that easily, but full tanking paladin usually isn't as helpful as a hybrid.