If you play long enough, you can usually get a feel for how long it takes for mobs and bosses to die in most content. No numbers required, just a feeling for how long it takes. You can sense roughly how long it takes for something to die, and you can usually get a good gauge on damage output if it takes too long to kill something.

Then there are the threat meters. Yes, it's not an exact gauge, but it can still give you an idea on player activity given the first rule of enmity generation: All offensive actions generate enmity, and dealing damage is definitely an offensive action. Of course the tank should be at the top of that list, otherwise damage out is the least of your concerns. An active healer (meaning they're doing both damage and healing) is usually second on threat because because they're doing two types of actions that generate threat, with healing (especially periodic healing) generating higher amounts of enmity. In the case of a active healer, DPS are usually closer behind unless in the event of popping a threat reduction cooldown (which is never enough to skew consistent results) or they died for some reason or another, which still justifies kicking them for constantly standing in bad because dead DPS means no DPS. Beyond those two reasons, if a DPS is miles behind the other non-tank players on enmity in the middle of a fight, that would indicate low activity, which would indicate low DPS output. If a healer is miles behind everyone else in threat, that would indicate the lower activity of just healing, which is fine as long as people aren't dying left and right.

And then there's another thing worth factoring in on these forums: voices on these forums are the -minority- of the player population. No matter how often you cite threads about healers whining about why they stopped healing, there are still healers healing and there are still people who want to play healers and keep playing healers because they find the playstyle fun.