Regen
 

Can you healers notice a difference between healing an iksar warrior with AC / regen boost to say a human warrior? In a group and at 51+ i mean, once regen starts to kick in. Both with same type of gear of course.

Races who have natural regen, Iksar/Troll, their health goes up while not getting hit.
Races who don't have that regen, their health does not go up.
(excluding spells/items/pots)

Spending time spot healing (not cleric healing) I notice their HP go up 1% occasionally.
Not having natural regen isn't detrimental but it helps. Especially if there is any downtime between pulls and they're not at max hp.
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You'll most commonly find people arguing over regen vs frontal stun immune. And those debates go in length depending on the class in question. For warrior I think ogre is king but I've seen iksar do just as well.

Druids and shaman probably notice more than clerics

Personally I've never noticed the race/regen of whoever I'm healing, but I think that says more about my powers of observation (and perhaps the power of Torpor) than anything else.

it probably matters least with a class like warrior which will almost never solo, and especially with complete heal.

and to correct an earlier implication, regen is always active, even in combat. It's just more effective when sitting.