I don't have much to back this up but I think that a spec's most favored secondary stat is going to tell you how well their DPS scales with gear. Crit in particular is a more punishing secondary at lower gear levels. For example, a Fire Mage with low gear (maybe 20% Crit) isn't going to perform as well. Crits make up the core of your damage, without many Pyroblasts you don't have very powerful DoT's to spread around, and your Combustions will be less than impressive. It takes a lot of Crit to crawl up that ladder. The goal is ideally 100% crit but you won't ever reach that. I tend to be more aware of the 80% non-crit than the 20% crit. And when you're playing a spec that relies on Crit, it becomes apparent very quickly that 80% of your spells aren't jiving with the best parts of your spec.
I would choose a spec that favores Haste or Mastery, because I like that those specs seem to scale a little more linearly. It takes away the (100% minus what your gear gives you) part of the equation. Some specs focus on Haste breakpoints etc., so I can't speak to those as well.