The Basics of Trapping
There's two universal drawbacks to traps that are good to keep in mind. First, they have a short arming time. That means that you can't simply throw a trap under a moving target and expect it to trigger. There's a good chance they'll get out of the way, especially if they're player targets. You'll need to learn to set the trap ahead of time and lure your opponent into it. This is, of course, relatively easy for a PvE mob, since you can simply fire a Distracting Shot and lead them right in.
For a PvP opponent, this is a lot less easy, especially if they notice you laying the trap. You have to be a little more crafty, and learn to keep your trap between yourself and your opponent, giving them no choice but to trigger it if they want to get to you. You may also want to use another form of crowd control such as Intimidation or Scatter Shot (which will be an 11-point Survival in a future beta patch) to "hold" your target long enough for your trap to trigger. Sometimes, if you're lucky, you can move around enough (or use a /kneel macro) to fake your opponent into believing there is a trap where there is one, causing them to blunder in the trap you actually did lay.
Second, they are all on the same cooldown. That means that you have to pick your traps wisely. Be aware of the Frost Trap you're giving up when you drop that Freezing Trap, or the Snake Trap you're giving up to drop that Frost Trap, and so on and so forth.
This is the mainstay of traps, the icon and the centerpiece of any Hunter's group utility. It's not quite as dependable as, say, Polymorph, but when you're looking to squeeze into a 5 man run, it can sometimes help get that final DPS slot if you mention you do have some Crowd control.
In PvE, it's best use when you can establish a good rhythm with your tank. When possible, have it down between 20-30 seconds before he pulls, so that your cooldown will pop and you'll be able to ready a new trap for when (not if) the trap breaks before the tank is ready to grab your mob -- be sure to put that second trap down far enough away that your target won't accidentally trigger it while still frozen.
For PvP, just remember the drawbacks. A Scatter-Shot/Freezing Trap combo could be just the thing for keeping that pesky healer tied up long enough for you to kill his partner -- and you'll probably want to trap the healer, since Priests and Paladins can just dispel the trap otherwise if left uncontrolled.
It's worth noting that the Wrath Beta and the PTR are both featuring a new change to the Freezing Trap. It now takes up the function of the now defunct Bear Trap. It will not immediately break automatically on damage, but will allow a certain amount -- my estimate at level 70 on the Beta servers was somewhere around 1500, which will hopefully go up at 80.
This provides a nice little safety net just in case the DPS "forgets" to switch targets or follow the tank boxes. At the least, it'll give you a chance to warn the tank before the trap breaks, and it means a stray Curse of Agony casting won't be the end of the world. A friendly reminder though: As of the current Beta build, it seems that pets still attack frozen targets (instead of automatically disengaging as they currently do on live), so be sure to pull yours off if you don't want your trap to broken in a few seconds.
Freezing Shot's dos and don'ts probably look about the same as Freezing Trap's. The trap it spawns still has an arming time, so you have to aim it where your target will be rather than where it is. In addition, the shot itself is a noticeable trail of frost in the air, so you'll have a hard time using it to trap PvPers. If they're watching for your kneeling, they'll be watching for the blue trail. You can see one Hunter's adventures with the shot in this video.
Of course, the shot may receive some more tweaks before Beta's over, so we'll keep an eye on it for sure.
This trap is somewhat overlooked by many Hunters, but it has a deceptive amount of power once you learn how to use it. If kiting is your game in PvE, this trap can take the pressure off of you for a bit. The ice slick should easily catch whatever you are kiting and give your Concussive Shot a chance to cooldown or yourself a chance to catch your breath.
PvP, however, is where this trap really shines. Rogues and Warriors have long been the bane of Hunters everywhere. Once they close to melee range, it can be hard to get them back out again. That's where the Freezing Trap comes in. Keep them on the other side of it, and they'll find a lot harder to get to you. Similarly, if you can get their healer caught in it, even if that pesky Warrior does get caught up to you, he may find himself missing his support when your own cavalry charges in. If you can learn to use this trap to divide and conquer, you'll have mastered one of the most important parts of Hunter PvP.
This trap would be a lot better if it wasn't so random, Since the type of Snakes it spawns are random, you may not always get the Mind Numbing or Crippling Poison you were hoping for. Still, it's extra damage, and may be worth throwing out in AE PvE if you aren't on crowd control, especially if you're up against caster mobs. Just watch out, stray Snakes may very well poison random mobs, making them difficult to crowd control.
Snakes can be helpful when a PvE mob has a random target ability. Sometimes, they'll target a snake instead of a PC, essentially wasting the ability. This trick made Harbinger Skyriss in Arcatraz a whole lot easier back in the day, for sure.
In PvP, it's nice in theory against Druids, Shamans, and Paladins. If they get poisoned by the snakes, they'll have a much harder time cleansing your Viper Sting. The trouble is luring them into the trap, as always. You may also find that you'd rather save your cooldown for a Frost Trap and leave the poison stacking to your pet Scorpid.
This trap is just not really worth it. The problem is, the cooldown is almost always better spent on another trap. I've long thought that they should put the fire traps on their own cooldown, so they could at least be used without locking out our primary form of crowd control for 30 seconds, but no dice so far.
If you're buff enough, it can be a decent DoT, and if you're a Survival Hunter, it will have a small chance to stun in Wrath. However, if you want a "stun," chances are you're better off with a Freezing Trap, and if you want the damage, chances are you're better off standing back and shooting.
If you're absolutely positively stuck in melee, you could technically throw down the Immolation trap, but chances are you'd rather be using your global cooldowns escaping from melee.
Explosive Trap fares slightly better than Immolation Trap if only because it is AE. If you're in an AE PvE group, or if you're trying AE soloing with a pet such as the Gorilla, you might as well lay down an Explosive Trap before the pull to maximize your DPS. If you're a Survival Hunter with some of the trap proc abilities like T.N.T., so much the better.
In PvP, I'd pretty much always rather have a Freezing Trap down for the slow, unless maybe you're pairing with another Hunter using Freezing Trap and have no expectation that you'll need to crowd control at all.
For me, one of the fun parts of being a Hunter is playing around with traps, learning the tricks of the trade and discovering the various ways to use them. If you aren't inclined to use traps much, or even if you are, I challenge you to experiment with your traps. If you can find ways to use them better, you'll be a better Hunter and one more desirable to groups in both PvE and PvP.