Leveling Specs

[Air]

For frost tanking, obliterate is for single target, and howling blast is for multiple targets.  Obliterate does more damage overall on a single target.  I will sometimes use rime procs to use howling blast on a single target to get extra threat.  I repeat, overall obliterate will beat out howling blast in damage on single target's.

For death and decay, not having the runes for obliterate / howling blast for your first rotation is perfectly fine.  It's all about getting the mobs setup first by using pestilence to spread diseases.  Than afterwards you can use howling blast and blood boil for aoe threat.  Death decay should provide enough threat to overcome initial healer threat, and give you enough time to build threat.  Also, DnD works just like consecration in that it applies a debuff on the mob once they get in it.  So if you need to move em, there still getting hit by it. 

For pulling I will generally try to death grip a mob into death and decay.  I try DG a spell caster instead of a melee type.  Even in heroics or raid's I will still DG a mob, just for the initial snap aggro.

One trick I use when aoeing tanking, is to just do: death and decay > IT > blood tap > pestilence > howling blast.
going over that, it would look like:
1) 1B1U1F
2)1F
3)1B > 1F or 1U aka 1 death rune
4)1B
5)1D1U (also, if specced for it, use death chill with howling blast for extra threat)

Biggest issue I find now is dealing with the gcd.  That aoe rotation I posted would require around 4 gcd's or 4 sec's to use it.