Just got to exalted on all my Shammies yesterday. To be honest, the dailies are a much more time-effective way of grinding your rep than MgT, though I did run through MgT a couple times to break up the boredom.

The quests in MgT only yield 250 rep each, IIRC there are three. One quest completion after the first boss, then the orb quest for which you need to get past Vexallus, finally the quest to kill Kael'thas. I didn't have too much trouble with the instance except that I could not kill Kael'thas. He was fine until the gravity lapse with the death orbs; with the just-keep-nuking strategy I got him down to 1-2% a couple of times before my guys all got orbed. I wasn't able to figure out how to effectively draw all 5 of my team away from the floating orbs before I gave up - I wiped about 10 times - grr.

On trash the standard 5 man shammy techniques work fine. In the larger groups, I would suggest dropping your grounding totems first and making sure their cooldown is up before initiating combat, so you can drop another set of 5 when the first set gets burned off.

For Vexallus, I spam the macro:

/target Vexallus
/target Pure Energy
/cast Lightning Bolt

In the courtyard before the priestess courtyard, you can creep along one of the sides and only clear half the mob groups.

In the courtyard with the priestess, I did not clear the two large mob groups behind the priestess. After taking her out, you can go straight into the building with KT inside.

The priestess encounter is pretty wack, but straightforward - burn the most dangerous guys first. I would use up most of my Earth Elementals on this one (don't really need elementals for Kael'thas) to give a little breathing room and remember to drop Tremor Totems - you will def. get feared.

The last big trash mob before KT is messy for pugs but not bad with focus fire and grounding totems.

As a reference point, I'm in mostly blues with a couple of epics and greens, about 680 spell damage at the moment.