The bad tanks don't 1) guard the healer even when asked to... 3) Never initiate combat to establish agro. .. I bring these up for purposes of comparison. As long as you are not guilty of the 4 points listed above then you are not doing a bad job, don't let some whiner get you down.
All good advice, Dak. I might disagree with #1 - when I tank, it's situational who I guard. If the healer is taking a lot of damage, I'll definitely guard them. But if they're not, I won't. Why guard someone who isn't taking damage? I'd rather put it on that dps that is stealing aggro or in aoe range. Just depends who is taking the hits. I'll switch if I need to.
#3) - There's a flipside to this one - I've seen tons of dps who have an itchy trigger finger, intiate combat, and just unload on mobs. They don't wait when they should. They don't manage their aggro. Granted - if the tank takes a really long time that's different. But generally speaking the tank should always be allowed to initiate combat. If they're taking too long then the group needs to speak up. I hate when you get over confident healers or dps trying to lead the group into combat. Silly. Know your place and your job in your group. If you don't know or aren't sure, just ask.