Why Beast Mastery?
As long as I've played my hunter, I've played Beast Mastery. Why?
It isn't because of the numbers, or the abilities--although I've learned that regardless of numerical disadvantage, a knowledgeable and determined player can top charts or get titles regardless of spec.
I rolled Beast Mastery because I liked the idea of it. The theme of being one with an animal partner is the reason I rolled a hunter in the first place, and BM really emphasizes that. Some players might choose the hunter class because they like the idea of stalking the wilds or sniping from a distance, but for me the idea of rushing into battle with a massive wolf by my side was too appealing to pass up. My other favorite classes have been shaman and druid, for similar reasons: a druid can shapeshift into beasts, and a shaman can take on the form of a ghost wolf to travel more quickly. And while World of Warcraft depicts these in very graphically simplistic form, the idea is there, and it's the idea that I love. I have two BM specs now: one for PvE, and one for PvP. They're quite different, but they're both BM.
How is Beast Mastery different? Well, the playstyle is entirely different in PvP, for one thing. Your focus is on pulling together a cluster of specific conditions, then unleashing huge burst damage. It revolves around positioning, micromanagement and keeping your pet alive and in the right place, doing the right thing at the right time. Your pet is an extention of your own character, and the extra control is a must. Anyone who claims that Beast Mastery is a simple spec has never played it to its full potential! I've come to love BM just as much for the challenge of getting the most out of an underused spec as for the theme of it.
In PvE I started raiding as BM back in WotLK. I found that once I'd gotten the spec, glyphs and pet choice (and talents!) just right, and completely mastered the rotation, I could outperform even the top raiders in every guild on my server's faction, single-target or not. Running ICC, I don't think I was ever outdone by another hunter--at least not consistently. My green devilsaur, Valak, and my gray worg Grim were my main raiding pets during this time, and I remember the competition (for BM hunters, anyway) between Devilsaurs and Wolves being very fierce. Part of the reason for my success, too, was probably my Orcish race--Command is nothing to sniff at for a BM hunter!
Anyway, the point is: raiding as BM in WotLK was difficult. You couldn't waltz in and do your part--you had to really crunch numbers, bring everything you could to the table and push to be the best. BUT, if you did put in the effort, you COULD succeed.
Nowadays things are easier. The mockery toward BM hunters still actually exists, I've found, in some unenlightened circles, but in truth BM now competes for top hunter PvE spec. The main difference between BM and the other specs in PvE is that your DPS (and threat) are split. You give less initial threat through Misdirection, and have to watch your threat far less while nuking. You can put your pet on one target and yourself on another, giving decent DPS to both. Heavy movement fights are somewhat easier on your numbers if you need to swap aspect, as the pet will stay on-target and continue with steady damage. On the other hand, a fight where you need to keep your pet on passive, change targets often or recall your pet to avoid damage will result in a much heavier DPS loss. If you're doing a lot of AoE, your numbers will be lower than a Survival hunter's--but if it's AoE now and then, and your BW is off cooldown, your AoE will be insane, as you can simply spam Multishots until everything is dead.
We also now have a lot more cooldowns to manage than before. We have Fervor to worry about, along with lining Kill Commands up with BW, trinket/racials and preferably Call of the Wild. Building up focus before a BW burst, properly timing Rapid Fire, even knowing when to pop Distracting Shot + Deterrance and then feign in an absolute emergency (and how, so you don't wipe a group or raid with positioning)--these are all key to becoming a good BM hunter. Granted, not all of these are BM-specific--but they're all important.
On the PvP front, the differences get more pronounced. As MM, you have a lot of control and a lot of damage--but it's more glass cannon-esque. You have Readiness for two Deterrances, sure, and Chimera Shot. You also have double silences, and that is an extremely good advantage against any caster, but healers in particular. BM has a much tougher time, in my experience, taking down healers--but on the flip side, I've never died to one, either. Spirit Bond and a Spirit Beast's heals are pretty much all you need to be a force to be reckoned with, and the steady heavy damage from a geared BM hunter is rather scary to many healers (I've been on the receiving end, too!), especially when paired with a good melee dps. A Kill Command can finish off someone who's dipped out of your LoS to heal, and Kill Command and the pet damage in general makes pillar-humpers far less of an issue (although the issue is certainly still there). The pet of a CC'd BM hunter will still put out a decent amount of pressure as well.
BM, though, needs to be extremely judicious about cooldown usage, and has to learn to be good at timing CC with burst. As BM you also need to know when and how to play defensively, kiting and using cooldowns to turn the fight around rather than going for pure damage. I think that preventing the enemy from hurting you--through CC's, stuns, kiting and the use of defensive cooldowns--is probably the biggest key to successful BM--and, really, overall hunter--PvP.
BM suffers from a couple weaknesses, of course. We lack the control of the other specs, for one thing, despite having the extra stun and periodic burst. Our sustained damage is lower than that of MM. If our pet is CC'd, we don't just lose Master's Call and the pet's talented or family abilities--we lose half our damage and a stun to boot, as well as any procs from the pet's abilities. That said, if we ourselves are CC'd, the pet is still out there putting out good damage--and preventing the healer from drinking (assuming the healer isn't abusing sit bugs, although a good Kill Command crit will make them think twice about doing this). Another weakness is fighting Death Knights--a good Death Knight will use Death Strike on your pet, meaning that you either need to out-damage his healing, or failing that, pull your pet back and kite and kill the Death Knight without a pet at all. Both are quite doable, but it's not a cakewalk by any means, assuming the DK is any good.
I think the biggest weakness, though, for a BM hunter is...
The bridge in Blade's Edge Mountains. If you're attempting to kill someone there, you can forget it; if your pet chases them up there, they can simply jump down and have a good few seconds with only half of your damage on them, and no chance to stun them. By the time the pet catches up with them, they're back on the bridge jumping off again! You'd better hope it's not just you versus them at that point, or the fight quickly becomes an exercise in massive frustration, regardless of kiting & traps.
Screw you, BEM Bridge. Screw you.
But yes--there's my take on BM, why I play it and where I think its strengths and weaknesses lay. I find it an impressively fun spec, and quite a fun class overall--but it isn't an easy spec to play to its full potential.
P.S. I had relevant screenshots, but posting pictures of Recount numbers seemed a bit much, so enjoy the random pet + hunter screens instead ;D