Marendyn (my feral druid) was fairly straightforward. For bear gear, I focused my evaluation criteria on how much damage I could take in melee, factoring together stamina, armor, +defense, +dodge, resilience, and so forth. (So far, I've just been assuming melee fights for this calculation. For resistance fights, I'd probably evaluate a separate set of bear resistance gear.)
For cat gear, I focused my evaluation criteria on how much white DPS I could achieve, including the effects of +crit, agility, strength, attack power, feral combat skill, et cetera. I have assumed that the DPS from special attacks scales with that from normal attacks, though I haven't actually checked that assumption.
Mazumok (my hunter) has been a bit more tricky.
My first step was to calculate ranged white DPS the way I'd calculated melee white DPS for Marendyn. (Interesting note: for my Marksmanship build, the attack power / crit tradeoff seems to prefer a lot more attack power than I had expected to find.)
But I'd realized that being able to use mana was important to my enjoyment of Mazumok, so I needed to take int and spirit and mana/5 into account. After some waffling, I borrowed an approach from the Healpoints addon: to account for magical damage, I take a weighted average of the magical damage I can do over one minute (to consider burst damage) and over five minutes (to consider sustained damage).
(Aside: by that standard, the best sustained-DPS hunter weapon is the very plain-looking Survivalist's Pike; weapons that provide int and mana as well as other hunter stats are very rare. But I use Terokk's Quill instead, because it looks much better.)
But that left open the question of stamina and other defensive abilities. Survival is clearly useful, but it's not clear what the tradeoff is between damage production and survival abilities.
I decided on the following criterion:
For a normal one-on-one fight, my goal is to bring down the foe before my pet dies.
Mathematically, that's true when (Pet modified health) / (foe DPS) > (foe health) / (DPS we produce).
Algebraically rearranging that to isolate all the factors I can control, I get (Pet modified health) * (DPS we produce) > (foe health) * (foe DPS).
So my criterion to maximize is (Pet modified health) * (DPS we produce).
I don't really have an intuitive way to describe that quantity; its units are health^2/second.
(This criterion is still vaguely reasonable for instances, because my pet acts as an off-tank occasionally, and sometimes Maz has to take a hit or two himself instead of letting it go to something more squishy.)
Now I'm thinking about doing the same for Amalefica, my frost mage. And again I'm not sure what the criteria should be.
The simple model is probably just casting Frostbolt repeatedly. I can certainly calculate the effect of +spellcrit and +damage on Frostbolt.
Intellect and mana regen are a bit more complicated. The tradeoff there might be evaluated by the question 'how much damage can I produce before going out of mana?'--or perhaps by the 1-minute/5-minute weighted average that I described for Mazumok.
The effects of stamina are also complicated to evaluate, because so much of my playstyle with Amalefica depends on avoiding being damaged at all. If I'm being hit, that interferes with casting spells and quickly burns through my limited health pool--my comfort zone definitely depends on never being hit. But stamina still has some value... I'm not sure how to manage the tradeoff.