One thing I really had to re-practice when Cataclysm was released was the fine art of a good pull. All through Wrath there was very little need to think twice about the way you decided to deal with a group of mobs because A: You'd probably keep aggro on them anyway and B: You could tank 15 mobs without dying. Alot of this changed in Cataclysm and I quickly noticed that I had really been slacking in terms of keeping my pulling-skills (if I ever had any) up to date. I remember actually having to use cc, los and other complicated tank skills back in BC, but that was such a long time ago. My first shots at marking and tanking around cc didn't go too well. Fortunately I did my mistakes in normals which rarely ended in distaster. Still, it made me notice that the fine arts of a perfect pull really is a school of knowledge in itself. There are so many things to factor in to make a pull a good one. There are still a couple of pulls I dread and very rarely get right, most notably the final pack of mobs under a lightning grid in Vortex Pinnacle. The one where you have to pull three casters out of the grid and collect them all, hoping your dpsers haven't already barged and/or forgotten all about your cc marks? Allthough I rarely wipe there, it's still a blow to my pride when I see mobs running around everywhere untanked. Here are a couple of things to think about when going about doing a good pull.
How you will deal with a pull depends alot on what kind of tanking class you are. Some classes have great aoe threat (paladin/dk), some have less good (warrior/druid). Some tanks have great ranged threat (paladin/dk), some have less good (warrior/druid). Some have a great setup of cooldowns for survivability (paladin/warrior), some have less good (druid/dk). Some have good cooldowns for threat (paladin, druid), some have less good (dk/warrior). If you feel like your aoe threat is an issue, you might want to cc just to make the group more manageable to you, not just to the healer. If you feel like you can survive a big pull by popping the right cooldowns you might avoid cc where other tanks would prefer some.
Identify your party
Make sure you know which kind of cc your party is capable of. If you're unlucky you've ended up in a group with a warrior, dk and priest dps, which gives very limited cc possibilities. Fortunately this doesn't happen often, most groups have at least some sort of cc at hand. At first you won't know if your cc actually knows how to cc, so it is possible that although you have classes that can cc, you don't have players who can cc. Still, the first step of a good pull is to recognize what your party can and will do when you do your first attack. This doesn't just include cc, this also includes charging head-first warriors or druids, trigger happy hunters or aoe-happy dpsers overall. Knowing whether you will get help from your party or have to struggle against them will do alot to how you would want to do a pull. If you can't hope for cc, you won't have to mark. If you can hope someone else will deal with a certain mob (silencing, ccing, killing solo) you might change the way you mark. Another factor that makes a big difference is the amount of dps your group has, or how good your healers is. The better the healers and dpsers are, the less likely you will actually have to think much about your pull at all. Just as in Wrath, you can worry less about cc and just barge in head first. If your party dps is low or your healer badly geared/skilled, you will need to think alot more before a pull. Some of these things aren't things you know before you've already done a couple of pulls unfortunately. Therefore it is generally good to use the first couple of pulls to test your group. Try setting up marks and see if people care to follow them. Try to put up less marks and see if your healer can still keep up. This will determine how you deal with the rest of the pulls of the instance.
Identify your mob group
The next steps to good pulls are more situation to situation based. The first of which is identifying the group of mobs you're about to pull. Depending on the quality of your group, you must now decide how much cc you will need and how to go about doing the pull in itself. Are there many casters? Are some mobs more dangerous/annoying than others? In some cases, like with the VP-healers, you will want to cc at least one because keeping two healers up is a real nuisance unless your dps is awesome. They heal themselves and eachother for nearly full health, which means that even if you have no trouble surviving the fight, neither do they, making the fight take ages (this has happened to me a couple of times). Some mobs do tons of aoe damage, while others might cc your healer which could end in tragedy (unless you have dpsers who can and do remove that).
Identify positioning and surroundings
Look around you, in what way can you use your surroundings to ease up on the pull? There are a couple of things to decide on. Do you want to tank them where they are or pull them back to a new location? Do you want to tank them by mob X or mob Y? This too depends alot on your party and the group of mobs, but also on what kind of tank class you are. Warriors and druids that generally have more trouble with ranged mobs, usually prefer to line of sight multiple casters to have them gather in a manageable group (unless they can all be ccd).
Identify the first strike
Once you've decided if and what mobs you want to cc, you need to decide how to do the actual pull. Do you want the cc to make the pull or do you want to do the first attack yourself? It is important to note that it is not always the best idea to use your initial attack on the mob that is supposed to die first. There are plenty of reasons to why you would want to use your initial attack on a mob other than your skull marked target. Although it might sound like this would make your tanking more troublesome, it is actually to make your tanking less troublesome that you want to use this tactic. Say for example that you have a group of mobs that are spread out. You want to barge in where the most mobs are standing and start doing your aoe threat, but also keep aggro on the mob that isn't standing in the group. Your initial attack could be on that lonely mob, making sure that he doesn't immediately run towards the healer when you tank the rest of the mobs (because he will be out of range for your aoe threat). One example is the pack of casters just after Corla in BRD. Another example is yet again the last pack of mobs in VP (before the stary things by the boss). If you don't have cc for all the casters you might want to use your initial attack on one caster and then attack the other, so to keep aggro on both.
Pulling is a huge part of the tank role. Most tanks absolutely hate it if they don't get to do the pull themselves, because it really makes alot of difference to how easy or difficult a tanking situation will become. As you notice there are loads of things to think about. Unless you see this as a fun challenge, at least to some extent, tanking probably isn't for you. Fortunately (?), cc is becoming less and less needed even in heroics now that people are getting better and better gear. Personally I feel like there are few things as rewarding as a succefully executed pull. On the other hand there are few things as frustrating as a utterly failed pull (which doesn't have to be just the tanks fault). Don't let this scare you off however, you quickly learn exactly how to deal with an instance and every pull in it, with only minor adjustments to accomodate your group setup. Eventhough tanking always is a challenge, many parts of it quickly becomes routine. How to deal with groups for a good pull is one of those things.