Rares and Champions...

[Roland]

Desecrator: Creats pools of green poison at players' feet

Actually they're red, like molten lava.

Extra Health: Mob has extra health. How much? Double?

Yeah, seems to be double. Don't think it's triple - yet.

Horde: ? (Thrall comes to help them?)

Seems to have a larger minion pack than normal. Not sure if they summon more (ala Illusionist), but the pack itself is definitely larger at the start. 'Course, they die just as quick.
Edit:
Ok, seems they also have massively increased health. Not 100% sure on this, but pretty sure as it took me far longer than I'm used to to fell a minion group of Horde monsters. So, increase health AND numbers? Increase in pack size seems minor, so this ability seems sort of weird...

Illusionist: Casts a version of Mirror Images. I had a solo champion that spawned four illusions that had a quarter (?) of his own health.

Knockback: Knocks players back on hit

Also slows you for about 3 seconds. VERY annoying when combined with ranged enchantments (Mortar, Arcane, Frozen) because you can't dodge nearly as easily.

Missle Dampening: I assume that ranged attacks do less damage. How much less?

No, they slow the missiles as they approach the boss monster. This does not effect certain skills, like Impale for the DH, but arrow shots like Bola Shot will travel very slowly - think the Wizard's Slow Time spell. Troublesome for us ranged characters, but not impossible (as I said, Impale works perfectly - may be a bug?)

Molten: Spawns molten lava on the ground

Also leaves behind a fire orb that explodes for large damage after 3 seconds. Wait to collect that treasure!

Shielding: Mob can cast a shield that seems to absorb a certain amount of damage.

This is a nasty skill. It doesn't seem to absorb damage so much as negate damage entirely for a set period of time. Seems to be about 6 seconds, but not 100% sure. When the shield drops, hit them as fast and hard as you can.
Edit:
Shield does in fact absorb damage. Managed to drop one in about 2 seconds with some heavy DPS. So with proper damage you can drop the shields fast enough to not be too concerned, but it still is a nasty skill.

Thorns: Does damage back to an attacker

Actually called "Reflect Damage," but yes it's an Iron Maiden-esque enchant. Makes it tough for us with auto-fire skills like Hydra or Sentry when we're low on health. Be careful with these ones. They only seem to reflect about 10% of the damage you deal, but if you're like me doing 1k+ DPS that's a hefty hit over time.