DK's seriously needed the buff, before the buff I literally had to stop white swings to avoid pulling off our DK offtank in some fights. Only tank I've ever had to fight for AOE threat was our pally and no bear should expect to have better AOE than a pally. (plus he is geared and very, very good)
Single target threat I have never had a problem with another tank pulling off me after a swap the problem has always been the other way if I zone out and start a full rotation up rather than just keeping my debuffs active, which I admit can happen a bit on farm bosses.
I agree with the several posters who suggest looking at talent build and hit/expertise. I've noticed a lot of tanks ignoring one or both of these stats then wondering why they have issues holding threat.
A point to note is that tank swaps are a two man deal, the new tank has to taunt and generate aggro, the previous tank should be paying attention to omen and not pulling ag. It's not that hard to control, going full burn on a transition is poor tanking.