2.) DK's are capable of both high armor and high HP, I have higher than warriors with similar level gear already, and my armor is about to get buffed significantly (in my gear, 3.0.8 adds about 4k armor to my sheet).
3.) If you think DK single target threat blows, you need to go meet some better DKs. I tank bosses frequently, and I've managed to surpass Arms Warriors, Frostfire Mages, Hunters (who don't waste CD's on FD), and Rogues (who don't remember they have a Feint button or aren't wasting the GCD on it), who are pushing 4500 dps. In 25-man raids I can now sustain above that on TPS for boss fights. It's the player more than the class, now more than ever. A skilled warrior, pally, or bear with equivalent gear will match my pace just fine, but DK's are not somehow left behind.
4.) Going back to something you posted previously, Pally threat does have a particular advantage since Seal of Light counts towards their threat, however it is neither consistant nor reliable because overhealing does 0 threat. Only encounters where there is raid damage will this provide a threat buff, and only proportional to the actual amount that it heals (at half the healing converted to threat). There is a reason they're fixing it in the next patch, but it is not universal, nor do any pally tanks I know actually depend on it.
Riddle me this, what DKs have you been running with that have been giving you such a low opinion of our class? Or are you just going on what you've "heard?"