AGM =
http://www.wowhead.com/item=19024
To get it it, you need to loot the Arena Trinket from the Chest that spawns every three hours, seven days a week, twelve months a year once to start the Quest that requires you to loot eleven more to get this trinket.
The Shield already went down quickly in 10-19 during WotLK, since Cata even quicker; it's mainly the Stamina that makes it attratctive. Of course, 20's already have access to a +4 Stam trinket, so it's +8 Stam + a long CD, flimsy-ish shield people are butting heads over, though Humans and those who choose to forgo an Insignia can get an additional +12 Stam out off it.
One could argue the AGM has more significance as a status symbol, as exampled by the 'not yet BiS so not worthy' remark made earlier by one of the posters. Earlier this year, my stance was (and is) that it is the effort that takes to get it that makes it significant: someone who is patient enough to get it, is more likely to weather difficult battles and to keep his/her eyes on the ball (as - despite the whines of many an end-cap toon - the whole objective is to Loot that Trinket, not a death-match).
As for the matter at hand: geez loueez.
On my old homerealm, at every time of the day (and night) you were likely to have other contenders - yet every legitimate Twink got his mitts on an AGM eventually.
Why?
Simple rules: Need before Greed, Twinks before dinks, Brains before Brawn
You want to get those Trinkets ASAP - so fighting over them when Twink A has 8 and you only 1 is silly, you help out A at best he'll help you in the future, at worst he'll be gone because he's done.
If the other groups show up, all Twinks pile atop of them, and the one who can Loot the chest does so - waiting around when capped toons are about is unproductive, interupting a fellow twink when capped toons are about will likely result in no Trinks for Twinks.
You don't want high-level toons lurking around, as this will only make other high-levels come as well. You CERTAINLY don't want to pay for them to be there, as this only gives them even more incentive to come there.
Let me repeat that: you don't want capped toons around, as that only leads to more capped toons in the Guru. You want Orkus OUT of low-level content, not in.
You don't want levellers to get the Trinket, as they'll level so fast that by the time they get the AGM it has become worthless.
The cappers you tell how silly it is they lurk around in low level content etc., the second group you can reason with. If that doesn't work, outplay them either by wits or by brawn. The question isn't IF you'll get an AGM, it's WHEN you'll get the AGM, and if someone lese Loots the Trinket it only means they'll be gone sooner.