If you have tunnel vision, please don't play a tank. :(

[lxSch]

I'm mostly talking about tanks that don't use flash/overpower and just attack one enemy, and then they all go after the healer once one heal goes off.

New/bad healer issue, regardless what tank is. I don't heal the tank until he builds enough enmity on all targets, if he dies before that - his own fault, should have positioned and flashed/cycloned faster.
DPS can choose to attack or not and noone in team will die whatever their decision was, i cannot see how this can be an issue for dps.

I had a PLD last night in brayflox, would pull as follows:

run to group of mobs, stand there not doing anything until one noticed him and they all rush him, then flash ONCE, and focus on one mob. now as BLM, I throw thunder first on everything to have better chance of proc, so that was good times ..

DoT does not break sleep (at least for ACN), i think casting thunder and then sleep would solve the issue. Sleeping targets do not build enmity towards healing, good WHM will cast sleep if tank has marked the targets.