While I'm gravitating a bit more toward making Havoc my main spec (again, we'll see about queue times,) I did take my Demon Hunter into a heroic, specifically Shadowmoon Burial Grounds.
Now, of course, given that Demon Hunters come out of their starting experience with 680 gear and with all the invasions I've managed to get up to over 700, one is already vastly overpowered for a dungeon that only even rewards 630 gear.
So surviving wasn't really much of a challenge.
The main thing that I noticed is that you're really going to be using Soul Cleave to burn off Pain, often if not most of the time absorbing less than the max 5 Lesser Soul Fragments. Demon Spikes should be your higher priority for active mitigation (unless your health has dropped low) but its cooldown is going to make it hard to use frequently enough to avoid capping Pain.
In terms of threat, I found that Infernal Strike was a fantastic way to pull - just hop in, pop Immolation Aura, and maybe a Sigil of Flame, and you're going to have your enemies' rapt attention.
You can use Throw Glaive (there's got to be a better name for this. Glaive Toss would have been better) for snap threat if you have something coming after a party member, and if you keep a charge of Infernal Strike at the ready, you can go rescue your healer pretty easily.
Nothing was damaging me enough to really warrant blowing major cooldowns, so no Metamorphosis. I'm still trying to figure out how frequently to use Fiery Brand, but I think with a 1-minute CD it's a "use liberally" kind of defensive ability. And the fire damage is not insignificant.
Overall, I think Vengeance has a ton of great threat tools, so until you're raiding and have to deal with other tanks, you're going to be pretty much set when it comes to that. Your significant self-healing will also make you slightly less reliant on your healer (something I really miss on my Paladin) but I don't think it's quite at Death Knight levels. But I definitely think that mobility is going to be a real strong suit for Demon Hunter tanks.