This is the go-to spec:
Spec updated to correct spec, copied wrong link before
http://www.wowprovider.com/?talent=1121 ... 3305d3103f
If, trueshot aura is 100 RANGED attack power, like it was at some point in vanilla, then only one hunter need take it.
Imp Conc shot and scatter shot. These are almost never used in PvE, however their alternative is also near useless. These two will help farming/5 mans/pvp. Otherwise you can move them into Efficiency and imp hunter's mark. If your guild uses hunter's mark (depending on debuff slots and number of hunters), one hunter should get 2 or 3/5 (no scatter).
Efficiency isn't needed due to mana pots and tubers.
Rotation
Your rotation will always be Auto, Aimed, Auto, Multi, Auto, Auto, Aimed, Auto, Muli
You do not want AimedShot or MultiShot to be off CD and not used. You also don't want to spam multi-shot while aimed shot is casting because MS has a hidden ~.5 second cast time which will delay your auto after the aimed and might then delay your next aimed or cause you to miss an auto depending on weapon speed. Do not use any stings, there are not enough debuff slots.
Further, following a proper rotation is what prevents you from having mana problems. Spirit regen kicks in after 5 seconds of not using mana. This rotation keeps all of your mana costs within ~.5 seconds of each other (aimed shot finishes casting and multi is fired). This allows you to regen with spirit for approximately 4.5 seconds.
Gearing
Armor is agi/ap/crit, usually it's pretty obvious what is an upgrade since crit/ap is rare on a piece with agi. The big thing though, is your weapon.
Weapon speed is far more important than weapon dps.
You need the slowest weapon you can get. Following the above rotation, you can see you get 3 auto's per rotation. At time point 0 you fire your first auto then start the aimed shot. While aimed shot is casting, you cannot auto but your auto-shot timer is counting down. Assume you have a 1.5 second auto attack after quiver, you lose a full auto when you aimed shot. Meanwhile with a 3.0 second auto attack, you miss 0 auto attack time. Then you have 6 seconds after aimed fires before you can launch another aimed shot. Due to this, you want a weapon with an auto attack time that multiplies evenly into 6. If you have a 2.25 auto attack timer, you 'waste' .5 seconds after your second auto before you aimed where you are doing nothing.
Following that, you should be able to see how Crossbow of smiting is the best vanilla weapon for hunters. It is even better then the Nerubian Slavemaker off of KT for certain gear levels, though I don't remember if they were attainable levels. Essentially, the .2 slower weapon gets 1 second worth of AP extra per rotation, at 306.6 ranged AP smiting has the same dps as Slavemaker, though still needs to make up that 1% crit.
Consumables
Unlike live WoW, quite a lot of consumables stack in vanilla wow. Unfortunately, most do not last through death and take some time farming though feign death helps with this. This is why I advocate for a slightly PvP build, losing out on 1 point being put into scatter shot and taking 5/5 imp conc.
Ground Scorpok Assay
This is a repeatable quest in the blasted lands. It gives you a consumable that grants +25 agility for 1 hour or until death.
http://db.vanillagaming.org/?item=8412
Elixir of the Mongoose
Best agi potion around
http://db.vanillagaming.org/?item=13452
Flask of the Titans
Since there is no agi/str flask, if you want to help out you bring this.
http://db.vanillagaming.org/?item=13510
Chromatic resistance doesn't stack with mark or pally/sham resistance aura from my memory.
Grilled Squid
This is only available in the winter. You cannot fish them up anymore after March 20th. You may want to stockpile a few, other wise go with any 12 stam food buff until Dirge's Kickin' Chimaerok Chops is released.
http://db.vanillagaming.org/?item=13928
Whipper Root Tubers and Night Dragon's Breath
You can farm these in Fel Wood. They have a 1 minute CD and don't share CD's with potions (though my memory is they do share CD with healthstones). You should always have a stack of these to assist with any raid healing.
http://db.vanillagaming.org/?item=11951 and http://db.vanillagaming.org/?item=11952
What do hunters bring
When everyone is geared, other classes will out scale you. However, at launch not everyone is geared. Hunters are one of the easiest classes to gear up well. Your default PvE spec gets you 3% hit. My memory is that 8% is hit cap for bosses.
Your rotation never changes, other classes will make mistakes while raiding (ok, really just rogues/warriors. Mages spam frostbolt and locks spam shadowbolt.) You should be able to easily get through a fight without missing a beat even once.
Once BWL comes out, threat becomes an issue. Hunters have FD and can ignore threat mechanics. Even in MC, threat can be an issue with mobs knocking back/stunning the tanks or with shazz teleporting.
Range. I don't think any other class gets 40+ yard range on their attacks. Maybe fire mages. You can outrange some mechanics like Shazz's arcane explosion.
Distracting shot. With a short range taunt for warriors, it isn't uncommon to assign a tank to a hunter, hunter distracting shots mob back to tank if he loses threat or on the pull.
Tranq shot. It's just a boss mechanic, they enrage, you tranq. Need hunters for this.
Kiting. Razergore is the first that comes to mind, though you can assist with Ony whelps with a frost trap.
Jumper cables. Raid wipes might be a common thing again. There are 40 people to rez. Trash respawns fairly quickly. No mass rez. If you are deep in MC and wipe, a hunter FD'ing into a jump cable can save the day if trash has already respawned.
Off tank! Not really. Hunter named avatar tanked Nefarion once as a gimmick. You can take a hit better than clothies, or deterence tank em. Such as warlocks banishing gar adds, banish resists you can distracting shot/deterence to buy lock some time to rebanish.
Edits
Posted proper spec, not spec taken from guy in 'Valid PvP spec' thread.
Added consumables.
Removed DM trinket due to not being in the game and myself confusing it with BWL trinket.