No one watches threat.
It is expected that DPS can go all out the instant something is pulled.
Why? Tanks got buffed out the wazoo (and no you best not look at my wazoo since swipe is now 360 degrees). Content in the end of BC and WotLK was an easy AOE fest; it wasn't until recently that DPS was capable of pulling of a tank exhibiting a base level of competence.
It's fun as hell though! DPS can be proud of their damage and tanks can be proud that they stayed in control.
I'm sure some tanks still prefer/expect the old ways of doing things where the onus was on the DPS to not pull anything, but not me. That was boring. Not as boring as early WotLK where threat was not an issue, but still pretty boring.
So that is the best part of tanking for me, "naughty" DPS.
What is the worst? Well the same thing. Sometimes boring is good.
While DPS can decide for themselves to take it easy when the mood strikes them, tanks and also healers cannot. Tanks and healers are at the mercy of the rest of the group.
So that is the current responsibility of DPS; not to watch threat on the mobs they are attacking, but to watch their threat level with the tank. Knowing just how much you can get away with on any given day.