Hey, nice thread with some good replies, I just felt the need to stick my 2 cents in and make my first post on these forums.
People are right about the general priorities of playing an enh shaman, and it is a matter of priorities, rather than a fixed cast sequence as many other classes have. However I'd like to amend some of the previous statements and possibly provide some clarification on when to cast what.
When / where to put your totems: As a melee dps class, you should be pretty close in behind the tank as he starts the pull, try to follow him as closely as you can. Most competent tanks will group up the mobs for more convenient AoE, try to drop your totems so your magma totem is within 8 yards (it's range)of the tank, remember, your fire totem is the front left totem to be put out, so often it pays to be to the right of the tank as you go in. If you get the totem placing about right, it should give you enough time to have your GCD refresh by the time you get within melee range. Dependant on the tank.
Cast rotation: Starting from Fresh - I'll assume you have LS buffed at full and no maelstrom procs up. Potentially, you have the ability to pull aggro so do the tank a favour, have him as focus target and try to attack his target.
The basic cast priority is:
2: Earth Shock
3: Lava Lash
4: Fire Nova
However, this is assuming you don't have any Maelstrom weapon procs up. With a full 5/5 of procs up, Maelstrom procced spells become your number one priority to cast. For groups of mobs, Chain lightning if it's off of CD, or lightning bolt for single targets or if CL is on CD. Also, as a precaution against inattentive / undergeared healers, I have a self cast / focus target cast macro to cast Chain Heal close to hand.
With large groups of low health mobs, Fire Nova takes a greater priority, casting it twice in the duration of a single Magma Totem being the aim. My secondary aim to be to try and build enough Maelstrom procs to get off a Chain Lightning before the mobs are all dead.
Generally, I'll find that before Stormstrike is off of CD, I'll have a couple of seconds with ES and LL also on CD to either refresh LS or Magma Totem.
Also, I try not to burn SS if the mob is about to die and there are other mobs in the pull with relatively high health, better to use LL even if it takes a sec to become available, and have the SS buff up on another mob that's going to take sustained damage from you.
Euhm, as another poster noted, Wolves, BL and Shamanistic Rage are all pretty highly situational, try not to burn them on mobs that won't see their full effect, or if the CD is going to be unavailable for a boss fight. The exception being Shamanistic Rage if you get aggro, coz your DPS really suffers when you're dead, and unless you have the T10 2 piece bonus SR isn't going to affect your damage - assuming you're not oom, in which case, pop it and focus on the mob with the biggest HP pool to try and get it's full effect, you may not get this - it really depends on the dps in the instance.
As I understand it, Windfury Weapon has a hidden CD of 3 secs between procs, with procs from EITHER hand triggering the CD for BOTH hands. As such I only keep WF buffed on MH and have Flametongue buffed on OH.
I have noticed that my DPS in heroics does vary quite a lot, mostly depending on how much HP the mobs have, but also with how lucky I'm getting with Maelstrom proccing. At the moment I have 2 part bonus T9 and the same T10, and my DPS is varying from about 2.5k - 7k, depending on the heroic. In the heroics where mobs die before I'm through a DPS cycle, my DPS is atrocious.
Finally - remember shaman are not a pure DPS class! You're a hybrid and (imo) the best hybrid class in the game! Use it to your full advantage, if the healer or tank is weak, you can assist them both with judicious choice of totems and by using your Maelstrom procs to provide additional healing if needed.
Stay awake and aware, watch the whole fight not just your corner of it and keep half and eye on health bars, then even if you are not top of the damage tables, you will be an invaluable part of any party.