As a first step, I tested out my arcane mage to see how the rotation and DPS compared to what it was previously. It seemed like a good place to start - mages are reported to be doing fairly good DPS in 4.0.1 (perhaps a little too much in the case of fire) and I'm fairly familiar with the arcane changes and the glyphs needed. Straight off the block, without much practice, I'm doing comparable DPS with arcane to before, with my unfamiliarity with the new rotation compensated for by the fact that it's so ... absurdly ... simple (see above). Peak arcane damage is obtained by ... spamming one button. Vanilla frost mages and destro locks may remember that feeling. The only reason to stop spamming that one button is because you're running low on mana, when you have to move to a lower-cost (but still simple) rotation until you have your mana regen back. Then it's back to AB spam.
Next step is my warlock. Destro was reported to be doing pretty good DPS, so I thought I'd give that a try. Not so good - my DPS was half what it was before, even after checking gems/glyphs/enchants. Right now I do lousy DPS even with EasyDestro to help me.
My mage and warlock have fairly similar gear, and both specs are reported to be doing good DPS, so what's up? Well, let's take a look at the destro warlock priority flowchart kindly provided by The Cursed Gnome:
I think most people would agree this this is a whole bunch more complicated than the current arcane mage rotation - the reason why the same player in equivalent gear is doing way more DPS with arcane than destro is fairly basic - arcane is simply easier at the moment. Of course a lot of these differences will fade as I become more familiar with the destro rotation, but the core problem will remain: destro takes more concentration than arcane, which means it's harder to keep up max DPS whilst avoiding all the other hazards a raid might throw at you. Not impossible, mind, just harder: I suspect a high-end progression raider would have no difficulty with either.
All of which raises the question: "what skill level should you balance around?". Rather than turn this into a mage vs warlock debate (the two specs I discussed above were just the first ones I happened to test), let's consider two imaginary specs, red and blue, as shown on the diagram below.
normal distribution (sometimes known as a bell curve), to represent the variation in player skill. In this case, by skill I mean the ability to execute a complex rotation / priority system in a raid environment, with all the attendant distractions. The red and blue lines represent the DPS that is achieved by our two specs in a raid environment, assuming that they're balanced when played perfectly.
It's that "perfect play" assumption that causes the problem. Stick to it and the blue and red specs will be equally represented amongst the best players, but for the vast bulk in the middle, blue is clearly a better choice. Unfortunately, if you balance around the abilities of the typical player (or even the typical raider), so that the lines cross there, red will be significantly better for elite players and the blue spec will vanish from high end content.
The ideal way round this would be to make DPS scale equally well with skill (and gear), but that's a tricky task. In 4.0.1 at least, Blizzard aren't there yet. The simplicity of arcane makes it clearly a better choice for anyone below the elite, for whom the complexity of destro means that it delivers worse results in practice. This is purely a personal preference, but it seems to me that the "sweet spot" for complexity is roughly where fire and affliction are now. Arcane seems too easy and destro too complex.