I've seen tanks being all over the place though, not sure if that's just wonky mechanics, or they were just not so good at it.
Bit of both.
AOE tanking is difficult for the first, oh, 30 levels or so, for anyone who isn't a Vanguard/Powertech. Assassins/Shadows only have one AOE until level 26 (AOE knockback excluded), unless they go Madness (highly recommended, even if you want to tank). Juggernauts/Guardians don't get their second AOE until level 32. Until then, it's pretty much cycling targets and hoping people focus fire one at a time. When they don't, it turns into a massive cluster@#%^ where you run around and try to peel mobs off the damage dealers who in turn are trying to kite the crap they pulled. Reminded me of back when Swipe got nerfed, luls. *twitch*
Vanguards/Powertechs get so many AOE abilities at the early levels (Sticky Grenade/Explosive Dart, Mortar Volley/Death From Above, Pulse Cannon/Flame Thrower before or at level 10) that it's not a problem for them. Not to mention they've got spammable ranged abilities, so running around isn't necessary.
Guardians/Juggernauts can tank in PvP with a DPS build (due to mitigation modifiers from the skill tree), but it's not recommended for raiding as a lot of the mitigation depends on mobility and raid bosses generally aren't very mobile.
Edit: Christ, Reddit is lagging like a boss. Expanding comment sections takes forever and editing posts produce errors like it's going out of style. Edited, Aug 13th 2012 7:55pm by Mazra