Here is my raiding Enhancement spec of choice.
Basic Play
It's played by using Stormstrike, Lava Lash, and Earth Shock when the cooldowns are up and Lightning Bolt whenever Maelstrom Weapon is stacked to 5. In AOE situations use Chain Lightning but watch your mana, because it drinks it up.
Weapons
Should you use a slow Main Hand weapon w/ Windfury and a fast offhand weapon with Flametongue, or use 2 slow weapons both with Windfury? I've read that both playstyle do comparable damage (somewhere on Elitist Jerks). So you should use the best slow weapon available for your main hand and the best offhand you can get your hands on, fast or slow (not medium speed), and enchant it appropriately.
Gear
You should be wearing "hunter" mail that has AGI, INT, AP, HIT, CRIT, HASTE, and MP5. Armor Pen isn't very useful because of all of the spell damage Enhancement Shamans do. Don't stack spellpower because you will get spellpower from your attack power. Haste is also less useful for enhancement shamans because it will only help with your Autoattack and Maelstrom Weapon damage, because your other damage is limited by your cooldowns. In 3.1, Shamans (among others) will see increased benefit to their melee attack speed from haste, so this could change.
Hit Rating
Hit Rating
Because Enhancement Shamans are a dual wield and spell caster class, hit rating is VERY important. The consensus is that you should get enough hit rating to negate the chance for your spells to miss, this will also make Stormstrike and Lava Lash never miss. At that point, your autoattacks will still miss, but it's believed that you will gain more by stacking other stats at that point. Spells normally have a 17% chance to miss against a level 83 (i.e. raid boss) target. Remember that Draenei provide a +1% chance to hit, and that Shadow Priests and Boomkins provide +3% chance to hit to the raid. So you might only need 16%, or 14%, or 13%.
Remember that your attack power formula is STR+AGI+INT, and that AP*.3 = Spellpower. These provide the core of your damage.
Having Mana Issues? Stack more Int, MP5, and Crit. MP5 will help the most. Int will help you last longer but won't do much more help in the long term. Crit will boost damage and efficiency through clearcasting. Remember an OOM Shaman is a lame shaman.
Advanced Play
About Maelstrom Weapon. You should use a 5 stack of Maelstrom before you use Stormstrike or Lava Lash, because SS and LL can get you more Maelstrom Weapon procs. If Maelstrom is at 5 stacks and you use SS or LL, you may get a MW proc that is wasted.
I would begin each boss fight by going crazy, wearing Lightning Shield and casting all of the spells that you can, to run your mana low, and then use Shamanistic Rage to fill it back up, then run yourself back down low (not OOM) and switch to Water Shield, and try to hover at a good value until Shamanistic Rage is back, so it will fill you back up. This, of course, varies from battle to battle. On Maexxna, I would make sure that I have full mana and the Wolves are off cooldown before she goes into a frenzy so that I can maximize my DPS during that particularly trying time.
It should also be noted that enhancement mana regen will be greatly aided in 3.1, since Improved Stormstrike is going to be rolled into normal Stormstrike, and Improved SS will give a 50%/100% chance to regen 20% of your base mana when you use Stormstrike. It's worth nothing that SS costs 8% of your base mana , Lava Lash is 4%, Earth Shock is 23%, and Lightning Bolt is 17% (all numbers before applicable cost reductions). There seems to be some consensus on Elitist Jerks that the regen provided by Improved Stormstrike isn't needed by raiding Enhancement Shamans. I say that the regen provided by it allows the shaman to stack fewer mana stats on their gear and focus more on damage stats.
As your gear improves you can swap points out of mana conservation talents to more damage talents if your mana regen can handle it.
That's all I have for now.
