When waiting in queue it is mostly for a tank or healer to show up, but mostly the tank. Some players will sign up as healer (and sometimes tank) just to get in fast. When a “tank” signs up, they are usually overgeared, and think having the stats are enough, sometimes they learn the hard way that it doesnt work like that. As for the “healers” some of them have no intention to heal, and go in expecting you to be a sustain tank. Sustain means you as the tank need no healing, the only healing needed is if dps need some heals. Now sustain tanking unfortunately mean many drop the healer role, because they get frustrated with being told to DPS when they signed up to heal.
As for the aggro. E5 marks a change in mechanics, and for ppl just looking to gear up quick, an E4 pays off more. The gab in successful runs between the two, just doesnt make up for the gap in reward. Also there some high geared will pug low runs, so they can go fast to last chest, in hopes of an Agent drop.
Personally i think they should form a group and stop taking the learning experience away from the lower levels, which thankfully some do.
Either way you will meet people that overgear you. You can only do so much about the Aggro on your end, the rest of it is on the DPS to regulate their damage. If they go into lower dungeons, then they should be ready for a low geared tank.
Personally what i do to counteract aggro issues is go ST on my Power abilities, and always use 2, one per weapon. Personally a Chaos tank, which can help a bit, as Immutable offers extra hate generation. If you feel you can go lower HP, you can also allocate some of your survivability into damage. Likewise the high geared dps can allocate damage into survivability.