I think a lot of people don't understand what a "skill-cap" is. Imirak is very close to the answer. Skill-cap generally is a combination of three things.
First, the amount of buttons you can press in different ways (what Imirak mentions). Basically, if you can only press 2 buttons and still be indistinguishable from the other guy pressing 2 buttons then you have a low skill cap. If on the other hand you have a lot of buttons to press that can distinguish you from that other guy then the class has a higher skill cap.
Second, and this is more player specific, is your need to position well. In other words, if you can stand and turret, you have a lower skill-cap in this ONE area. If having superior movement makes a difference then the class has a bit more depth to it and thus a higher skill-cap. The best example of this is Death Knights in Season 5. You could literally just spam Icy Touch, Death Strike and Death Grip and be virtually indistinguishable from every other Death Knight because movement simply did not matter. You could force players to have to come to you or grip them to you every 30 seconds.
Third, and finally, off abilities and resource utilization. In other words, if you're a DPS class that functions off of a single resource mechanic (lets say runic power to maintain the DK analogy) that builds upon itself then you're always going to have a lower skill-cap then a class that has a resource that is constantly driven down and is hard to replenish. For example, a healer usually has a higher skill cap at the level cap meta-game because you can constantly do a better job of conserving mana.
Now, with that being said, how do you go about figuring out who has a higher skill cap? Well, you should take the aggregate of the above three and go from there. The best way to do this, however, is to simply think about the classes. You all play 20s a lot (as do I). There are certain classes where it's apparent when a far better player is playing. Moreover, there are some classes where it really doesn't matter who is playing, the result is always the same. If there's a huge range of skill in a class that usually shows that there's a higher skill cap than the other class. Two important things to take note of here; first, do not take into account if the class is underpowered or overpowered (in that skill-cap measures only the range that differentiates a skilled player from a non-skilled player and NOT the difference in each class to the other), and second, don't take into account player skills (in other words your ability to strafe or move well doesn't make the skill-cap of the class higher, it just means you can move better than the other person).
With that being said, I'll rank mine and explain why I think it is the way it is.
BM Hunter
Balance Druid
Survival Hunter
Feral Druid
Marksman Hunter
Resto Druid
Subtlety Rogue
Elemental Shaman
Holy Priest
Disc Priest
Resto Shaman
Frost Mage
Shadow Priest
Enhancement Shaman
Destruction Warlock
Arcane Mage
Combat Rogue
Ret Paladin
Demonology Warlock
Holy Paladin
Affliction Warlock
Arms Warrior
Fire Mage
Prot Warrior
Prot Paladin
Assassination Rogue
So to explain my reasoning. First, hunters should be very self explanatory. Hunters have the most abilities at this level, and the difference between a good hunter, a great hunter, an average hunter and a bad hunter is highly differentiated. There's a reason everyone can say Biggie was the best hunter without any argument because it's pretty obvious who is really good and who is not. Your ability to use your pet, pet abilities, healing your pet, dismissing your pet, kiting, focus management for burst, scatter, conc etc are all brought into play. Yes, you can complain that hunters get all these abilities but the fact that they are there means good players take advantage of them.
Druids go in order I place them (interspersed with hunters) because they too have a lot of abilities that good druids take advantage of. Feral is all about kiting with roots, healing others/self healing, and using pretty much every ability under the sun. Balance gets a higher skill cap, however, because they have to maintain mana while still using two different schools to be "good." In other words, a good balance druid needs to be healing, DPSing and doing so without losing all of their mana. And even if they do lose all their mana, they need to be able to go cat and start doing some feral damage.
Rogues get the slots they do because of again, the high difference between good and bad players. This is far more apparent in arena than BG's so I'll explain it to those who don't do arena. Rogues (Sub) have the most CC and control in this bracket. Good rogues should be getting saps off, restealthing (that's one of the few things that feral druids never have to worry about that rogues do), gouging, kicking and shadowstepping properly at all times. Good rogues become apparent because many of the abilities that distinguish good rogues at end game exist at this bracket (shadow-step kicking/gouging, restealthing with LoS). Unfortunately, most of these abilities come only with Sub. Mut is basically a bad warrior, and Combat gets a pass only because it requires ToT.
Shamans get third. Dankfury proves this better than anybody. Long story short, Elemental and Resto both have to purge anything and everything they can, ensure they are shearing, allow teammates to kite around Earthbind, DPS and heal, and generally be a nuisance. If you're Ele you get a higher skill cap because you have Thunderstorm and your ability to use it properly makes or breaks your team. Resto gets second because a good resto shaman has to take advantage of melee abilities while still kiting in between the 6 second CD. Enhancement is far lower because outside of using your negligible amount of mana for purges and heals you really are just spamming a few abilities.
Priest is next up. Priests have the ability to do everything. Good priests do a ton of damage, keep everyone up via shields and healing, fear well and still manage to not go OOM. Bad priests put renew, shield and cast a flash heal to heal the person who just lost only 200 HP. Those priests go OOM fast and then wonder why the other priest is not OOM. Holy adds an extra dimension since it gets a stun with Chastise. Disc gets an easier fake with penance. Also, if you're wondering why healer classes are up so high it's because unlike non-casters faking actually matters.
Mages are next. Frost mage gets the highest spot because you can't blanket CS so you have to connect, you get a pet nova, you have to be able to time shatters and you have to have good pet management or it will get two shot (in other words, good frost mages control their pet so that people have to choose between them and their pet (instead of like most frost mages who just leave their pet by their side so that they can be tunneled)). Mages have the second most control in this bracket and that's why they get the higher skill cap than other classes on this list. Arcane Mage is next. Arcane Mage is far lower on the skill cap (Fire doesn't even place) because it's fairly one dimensional. You get a blanket so you can use it whenever you need 3 seconds to kill something. You have insane mobility due to your major damage coming from an instant. Long story short, your skill cap is defined by how you use poly making it the lowest skill-cap of the casters by far.
Warlock is next. Lets be real. You guys are putting warlock on top because it's an underpowered class. There is nothing that distinguishes REALLY good warlocks from good warlocks. I can't tell the difference between warlocks at a certain point in arena (and if I can it's because of player ability (timing a fear to save your healer)). Warlocks get a spammy CC that's pretty easy to maintain and then largely tunnel damage or DoT everything in sight. Nothing that makes warlocks a higher skill in end game exists in this bracket. The only thing that puts this higher than the other classes is the fact that you can time burst to get kills.
Next up are paladins. Holy gets the top spot. You can tell the difference between good Holy Paladins and Ret Paladins immediately. Your ability to fake is very important (since you pretty much sit there if you get locked), and the good Holy Paladins are the ones that are 10k+ healing over everyone else in BG's. Judicious use of your CD's, HoJ, using taunt on pets to break out of CC etc all come into play here. Good ret paladins should be on par in healing with the actual healers while still contributing burst and damage. Use of Holy Power stacks still comes into play. Prot Paladin gets a low spot if you're a non-hybrid (so everyone but Acai) because you could be brain-dead and still be indistinguishable from the best prot paladins.
Finally you get warriors. Warriors don't do anything that's anywhere near a skill cap. You get no interrupts, no stuns, no gap closers, CCs etc. Basically the only thing that can make you a good warrior is how you get away for charges (and you pretty much never get to do that).