I have played all three specs on the beta and done at least some questing in all three specs. While it will always come down to one thing, which spec you personally enjoy the best, I am going to offer my personal observations and opinions of leveling as a hunter on the beta. I try to level effectively so the way I judge it will of course be based on that. Once again I will say, level as what you want to level as all three specs are viable for leveling.
Pros: With its long and powerful shots marksman can make quick work of mobs in just a few shots. Being it is the current best spec damage wise it is also the best spec to down the rares, seemingly hundreds of them, that you will encounter while leveling. For leveling, with thrill for cheaper aimed shots and stampede for rare mob burst marksman can really do a good job of making it a smooth and easy trip to 100.
Cons: With its long shots, while powerful, you could find yourself slowed down some. Until you get kill shot at 35% you would never have a use for that powerful and free cast either. The mastery, which is designed for standing still, goes against everything that has ever been held true for hunter leveling and that is that you keep moving all the time. Sure, you will not lose much while moving, but it still feels off to do so when you are on a class that should be standing still. Add to that the complete lack of instants, which make tab and shoot for mass tagging and pulling impossible, you do not really have many of the tools that make hunter leveling so fun.
Opinion: Leveling as marksmen will be slower than either of the other specs. If efficiency is what you are looking for, you will not find it here. Even as the top DPS spec it is not enough to even be worth considering as a leveling spec because DPS does not matter when the difference will be killing something in 13 seconds or 15 seconds. The power of the instant casts that the other two specs offer dwarf anything MM might have to offer by a long shot.
Pros: Your pet, duh. One of the biggest pros of being a BM hunter while leveling is that your pet is powerful, does most of the work, can hold aggro better, and can hold more mobs at once. With the mix of having instants for mass pulling, multi shot for beast cleave which will help your pet with aggro, and the powerful pet which kinds of sums up its usefulness, BM has always been a formidable leveling spec for hunters. BM also still has burst which will help with rares out in the world which you will run across a lot of.
Cons: Your pet, really. If you are a full normal (now heroic) mode geared hunter even at the least your pet can one shot mobs up to and occasionally past level 94 even in survival and marksmen spec. Imagine what it will do in beast mastery and if you have heroic (now mythic) gear. It will one shot more mobs. And do not even consider blink strikes as a skill while leveling as BM. Remember, if your pet kills something before you damage it you get no experience, no loot and no quest credit for the kill. Blink strikes along with your powerful pet will ensure that you are killing a lot of mobs and getting nothing from them. So your powerful pet might actually slow things down instead of speed them up if you are well geared to begin with in the first few levels.
Opinion: If you are willing to micro manage your pet, or are coming into warlords with an item level of 520 or so, BM can and will be the best spec to level through warlords with. It has all the pros mentioned and the cons won't be as bad. You might still loose some mobs here and there to the pet killing it before you touch it, but it will be less likely at lower gear levels and while trying to manage your pet. So weigh that with how you like to level. If you do not mind watching your pet like a hawk it is great, if you just want to kill everything in your range it might not.
Pros: From a DPS standpoint survival is dead last. It does not have the power of MM or the burst of BM. All of that sounds like a con right? Absolutely not. For leveling survival uses its shortcomings at 100 to its advantage. Even more so if you have the four piece set, which can and will last you all the way to 100 if you have the normal (now heroic) or better version of it. Survival has a fair deal of instants, has a steady and consistent damage flow and AoE DoT damage along with its pet being weaker than a beast mastery pet which makes it less likely to kill mobs so quick without you hitting it. This makes AoE mob grinding so much easier. Sure you will not have MMs power against rares or BMs burst against rares, but all rares out in the world can just as easily be soloed as survival even if it is at a more steady and even flowed pace without the big numbers or burst damage. Add to the fact that survival has survivalist which heals you whenever you kill a target that grants honor or experience. This will keep you healed up nicely should you take any damage.
Cons: You won't have any on demand burst, you won't be able to let your pet do all the work against the larger mobs, you won't be killing things as fast as you might be able to be killing them in other specs, you don't have any hard hitting shots, and you don't have any execute like kill shot. Looking at it from those stand points it sure seems like survival is lacking over all, which is probably because it is really lacking in all those things I mentioned.
Opinion: With many instants, many heals, and the luck of the proc on the four piece set, this makes survival the unequaled leveling machine when compared to the other specs. It can keep moving, it can keep pulling, and if it gets itself in danger and takes a little damage you never really need to worry about it. The pet can still hold aggro well enough with MD, you can still kill rares well enough, and you can DoT the world with all those instants which will make questing, and completing quests with all that competition so much easier.
Suggested leveling specs in Warlords:
Skills for all three specs while leveling (not 100):
Post Haste (speed boost), Binding Shot (control), Spirit Bond (healing), Thrill of the Hunt (more instants), Stampede (burst), Glaive Toss (instant)
1st: Survival: Instants, healing and DoTs.
2nd: Beast Mastery: Burst and pets.
3rd: Marksmen: Slow and strong.
What will you be leveling as?