How many alts to you have? How many of them are of high enough level and have had enough experience that you can compare your performance to others of the same class and spec? Now, compared to them, or according to your own pre-set standards, do you suck? Is the class/spec you are playing the "Perfect Fit" for you, perfect in that you can intuitively know when a proc from spell A has occured and spell B is ready to launch. You do it so quickly, so seamlessly that your performance is nothing to be ashamed of. In fact, you perform so well that you are recognized by your peers for outstanding achievements and abilities.
Or maybe you are just the person who is "perfect at everything"...gag!!!!! You can leave now, us mortals don't need you telling us how much we suck.
I have many alts, a Warlock, Shaman, DK, Priest, Mage, Paladin, Hunter; all at either 80 or high enough that they have seen their way around the world and have sufficient experience to make a judgement on whether these class/spec combos "click" with me or not.
With my Warlock I've tried all three specs. They are all DPS specs, so ultimately there's not a huge difference like there is with a Shaman or Druid, but the differences are still noticeable. I've settled on Affliction, and I'm not exactly sure why. Throwing fire out as a Warlock never made much sense to me, so perhaps this prejudice hampers my ability to play the class well. Demonology is fun, but again I can sometimes seem uncomfortable not really knowing if I should primarily use shadow or fire spells or both, so I watch my minion fight while I stand 30 yards away flossing my teeth and decide that I need to go back and do what's most comfortable...DoT and suck my way to victory. Hey, look at that, I do suck!
My Shaman is a different story. I made him as a male Dranaei, which for whatever reason didn't look like he'd be the type to be afraid and stand in the back of the room throwing out weak bolts of lightening all over the place. No, he looked big and tough enough that he was one who wanted to mix it up and smash people's faces as Enhancement. So that's what I did. On my trek to 80 I flirted with Elemental, but only at night when they other side of my brain was asleep and didn't know I was cheating in it. I didn't have the same life-sucking spells as my Lock, but I still found a way to suck big time. Later when my then small guild asked me to try healing, I did so reluctantly. I am now a raid healing Shaman, and a very good one, but I am so much better as Enhancement and make sure I do everything else in the game (daily randoms, weekly raids, farming, battlegrounds, dailies, dancing) in my Enhance spec. It's how I roll...and I definately don't suck at it!
I've had very similar experiences with just about every other toon I play with on a regular basis. In each case there seems to be one particular spec that I understand better than the others. One talent tree that makes sense, that I feel like I don't have to theorycraft to play extremely well. One that is a "perfect fit", in other words.
I've known others who have similar trouble. One girl I play with just can't ever seem to understand any melee DPS class. She reads, studies, gems and enchants properly but for some reason there is something that doesn't "click", and as a result she just sucks at DPSing with any melee class. Likewise for a fellow guild-mate who has tried to heal; most instances have resulted in many, many untimely and unwarranted deaths. The healing classes he plays he's just not good at. Which is fine, because he's an unbelievable tank, the best we have.
I don't know what factors someone's intellect, personality, or past personal experiences play into having certain class/spec combinations not fit them. I have no idea why one is a supremely awesome tank while the next, with equal gear and playtime just can't get it. There must be billions of permutations of variables that go into you or I being great at playing a Frost Mage, but others of equal gear and even knowledge suck at it.
I have no answers, just that everyone is different and that's a wonderful thing. If all of our crazy little personality traits were the same, and we were all good at the same thing, we'd all be playing WoW as an Enhancement Shaman, and for as much as that's a perfect fit for me, it's a good thing it's not for everyone else. Variety allows us to play in 5, 10, and 25 man groups, each with a different spec and different abilities that compliment each other. So thanks, ya know, for not being anything like me!