Things to keep in mind with Thaanatos' post. This has a lot of 3.1 in mind.
Nitpick: You linked a Frost build when talking about your unholy build.
I'll also preface this with It's all up to your style. If you enjoy the style of Unholy, then by all means, stick with it. I think it's good to know and understand all 3 specs, because there will be encounters (especially hard modes) that will really play to the strengths of one spec over another.
Anyway, I was Unholy for a while, and have since switched to frost.
Frost has more avoidance and less passive mitigation of the 3 trees on live. This will change come 3.1. 5% passive damage reduction due to the new BB on all specs, and you'll get another 2% should you go improved frost presence. I haven't tested it (but I think I will now), but you should still be able to Death Coil yourself with the new Lichborne. Fairly lackluster still, but anyway...
Sapphiron is a pain to tank as frost. I get higher resist/miss rates vs Sapphiron as frost than I do as unholy. Other than that, my single target threat as frost is significantly higher than when I was unholy. I can break 10,000 TPS as Frost (9000 for the entire fight.. probably more since I'm using a VotTW build), it was very difficult for me to do as unholy.
As far as the "snap aggro" thing, I can relate. I went to cover for Lore for a 5 minute (this was before the change) 10-man malygos, and they were blowing cooldowns as they were expecting two ranks of SoR + whatever else he opens with. Even if everything connected, they were ripping aggro off of me. This changes significantly with the change to HB in 3.1 - pre-pull pop deathchill, then open with a HB. DK's now have a SS/SoR equivalent to open fights with.
On another note, I think I'm going to do some research on the DnD+Morbidity as frost thing. My role officer and I keep having a discussion about it, and while for 3 runes it's probably the highest threat, it's only looking at a piece of the picture. Lack of RP from chaining Obliterates/Scourge strikes (assuming 2/2 Dirge/Chill of the Grave) and other things I think hurts that theory. Plus the fact that it's "clunky", and hard to fit in a normal rotation.
Luck is what happens when preparation meets opportunity. -Seneca
Everyone marvels at a square egg, but only the chicken understands the PAIN.