You can actually LOSE some Defense in favor of other gems if you want. You only need 535 for everyday Heroics (but I would just keep it at 540 anyway). Going above the uncrittable number isn't bad, but it is better left until after you get other things up.
Adversely, you can grab more Defense and replace SSG with Sword Shattering for 4% more Parry.
Other than that, I would just do my best to get my AP and Sta as high as possible without going under 540 Defense.
Do you know if your group will have a Shaman in it tomorrow? If it will, lose Icy Talons.
Either way, I would alter the spec to be more like
this, with that last point going wherever you want. If you are REALLY nervous about keeping hate, you can take Morbidity--but it just isn't that good. Using DnD every 15 seconds utterly destroys your rotations, and you can get similar hate out of an HB every 5 seconds (especially if you have 5/5 KM and 3/3 Rime).
And I always go for Epidemic when using diseases, because it makes my rotation times line up nicely. Normal diseases are 12 seconds. Epidemic makes it 18. That is enough to get all your HBs and BSs in, a Runic dump and then reapply them. Otherwise, you'll lose decent threat here.
People debate the usefulness of Hungering Cold for tanking. Personally, I don't like it--too many things are immune to it, and those that aren't resist like hell/break quickly. Plus, the minute a party member hits the thing, it unfreezes. Also, with FS and RS, you have enough stresses on your RP as it is.
You can take Rune Tap--I don't like it, but others swear by it. You regain 10% of your health (20% if glyphed, also giving 10% to your party) for 1 Blood Rune. If you use it in the same rotation as Unbreakable Armor, you won't get that screwed up--you'll still have 1 Death Rune and 1 Unholy Rune available the next time around for an Oblit or HB (or to put up diseases).
Death Chill is a decent choice, and using it at the start of a fight will guarantee enough hate you won't even need to touch DnD (after your IT is spread, of course). 80% of the time, I have a KM proc for that first HB, but it will last until your next one (and KM procs can carry over to your ITs). And 2 minutes is short enough you can likely fit it into (at LEAST) every other group of mobs.
You can put 1 point into Ravenous Dead, giving you an additional 140 AP for additional threat on your ITs and HBs.
You can also put it into Virulence, since missing SUCKS for DKs.
Acclimation is another option (I have no clue what instance you are doing, so who knows if this is a good choice).
As for Glyphs, go for Glyph of Obliterate definitely. After that, you have a lot of options. Glyph of UA is nice, but it isn't THAT much more mitigation. Personally, I'd take it, but it is up to you.
For the third, you can take:
Glyph of IBF, making you never have to worry about RP when using it.
Glyph of Frost Strike, which makes it easier to manage with RS.
Glyph of Rune Strike. More threat is more threat, but the numbers I have seen for this glyph aren't impressive.
Glpyh of DnD. If you take Morbidity, might as well make DnD better--you've already chosen to destroy your rotation.
Glyph of IT. 10 additional RP is nice, but you will only use this ability every other refresh, and only once. Not that great.
Glyph of Rune Tap. If you are taking Rune Tap, but don't want to take either of the first two glyphs, you might as well take this. 10% health to your party and 20% health to you is nice.
I'd consider one of the first two the most though. Frost Strike basically means you'll get to use it more often. IBF means you don't need to sacrifice a RS (or a FS) to use it.
For Minor Glyphs, take Horn of Winter, Pestilence and either Raise Dead or Blood Tap. Neither are that great, but BT is better if you are willing to give up a Bag slot and are worried about the damage you will take.
As for rotations:
Multi-target:PS>IT>Pest>HB>Pest>FS (if possible)>HB>Oblit>HB>FS (again, if possible), repeat.
Single-target: PS>IT>Oblit>BS>BS>FS>Oblit>Oblit>Oblit>FS, repeat.