I was a pretty defensive tank, when I had first started. I remember I'd number every single monster, and I'd get all pissy if someone killed them out of order.
That was three years ago!
It took me learning that:
1- Spreading DoTs is a (good) thing;
2- AoEs exist;
3- Not everyone likes having their hand held.
Though, in retrospect, really anyone who followed my kill order was holding my hand. Looking back now, I don't know why I had issues holding aggro.
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In response to the OP, well... The friends I had were either bad players or they were afraid to be mean by telling me what I was doing wrong - so I had to learn from the unhappy parties I had when they'd get angry.
In short, I wish my friends would have just told me earlier rather than thinking I was in blissful ignorance.