How much damage is enough

I realise this is kind of a how long is a piece of string question but nevertheless I'm kind of curious as to other peoples experience.

I'm a decent but not great tank. Average gear level says 350 or close. We haven't raided at all so I'm just in heroic drops and rep gear.

I generally keep aggro except when the boomkin opens up just as I'm manouvering the mob so you could argue that I do "enough" which is around 5-7k dps depending on the instance.

But I ran as lolret with our guild's DK tank, now he simply is a better player than I and I accept that, but his gear is roughly the same and he was doing 11k dps as tank!

It seems to me that I'm missing something. I can gear for more hit/expertise/str but that seems wrong in a way and I'm not sure an extra 2% hit will make the difference.

Any thoughts on what dps is ok? Is it just our lot as pallies or is it me?

Arneus on US Khaz-modan (sorry logged in ret gear will switch over when I get home).

Single target or AoE?
For single target 11k is very high, in heroics i think i manage 6k or so (its been a while now tho)
Aoe, say 3-5 targets, 11k is on the edge of low, id say 13k is very doable (hammer inq lol)

Exactly Single or AoE. Did a Grim today came out at the end doing 14k!!! But that was aoe all the packs, making sure to Hammer every 3 secs and trying to keep up inquistion.