Heirlooms can be enchanted only with enhancements that will not bind the item to the character. The items are treated as item level 1, and as such can not use most Burning Crusade or higher enhancements, which included Death Knight runes until 4.0.1. Also worth considering is the fact that some of the enhancements that can be used will not actually take effect unless the character using them is of the appropriate level. [Chart that lists enchants follows]
We recently tried a guild heroic run, and it was capital F fail, largely because we didn't have quite the dps we needed, but also largely because we only have four 85s and none are tanks. If we had a tank in-guild, I'm betting they'd be patient enough as we learned how to bring down each boss.
(Probably didn't help that we had three boomkin. Not the most varied set of buffs.)
Anyhow, I've been considering going death knight. There's no way I'm leveling from scratch to 85. I'm almost too lazy to go 55 to 85. But with the right heirlooms, things will go 20% more quickly. I recently heirloomed an alt that I let a friend play as we did recruit-a-friend, and the one +INT enchanted hammer on their druid made it seem like we were cheating.
I know what you're thinking -- just regear Jal to bear for off-spec. I'm considering it, but I haven't heard much bear love recently, and haven't run an instance with a bear tank yet in Cat. I'm betting there's a reason.