the thing is that even if you pick up tanking talents like better armor and stuff, with shield you as enhancement shaman will do ridiculous damage to do tanking through damage and your aggro generating ability frost shock wont be enough to tank through full nuke

and after all why would you need a tank for group quests. I dont know any untankable quest by elem shaman untill lvl 70. Tough high level quests can be tanked by earth elementals but usually I just nuke the hell outta elite guys and so on.

And one more thing. Enhancement shaman will be pretty useless for ranged dps group and will slow down your killing of mobs.