When we itemize armor and weapons, we nearly always give them a set amount of Stamina and a set amount of Strength, Agility, or Intellect. We call these the primary stats. The secondary stats include haste, dodge, crit, and mastery, as well as stats with cap targets, like hit and expertise. I'll discuss those, as well as Spirit, a little later.
We’ve seen a lot of players crunch the math and come to the conclusion that, for example, Strength is much better for them than haste. This is intentional. We want to know that you have enough health, attack power, or spell power to be competitive at your level of content. When you think about it, it also rarely affects gearing decisions. You're never really choosing between gloves with Strength on them versus gloves with haste on them. (At least at max level. We have a little more fun with the lower level quest rewards.) Likewise, you are rarely making decisions about whether to enchant something with Agi vs. crit. Now, you can choose to reforge items to decrease an undesirable stat, though you can’t remove it completely. You can also choose to socket all Str, Agi, Stam, or Int gems. However, socket bonuses are stronger in Cataclysm, so you might want to use orange, purple, or green gems instead, and hopefully the secondary stat on those gems becomes an interesting choice.
We hope that secondary stats overall are just more interesting than the primaries. While Strength or Agility will make you always hit a little harder, critical strike rating makes you sometimes hit a lot harder. Haste lets you do more (whether its providing resources for melee or speeding up caster spells). Mastery makes you better at what you do. We want your choice of secondary start to mix up your playstyle, if only a little. Unlike primary stats, we want those secondary stats to be a choice. If one is far better or far worse than the others, then we haven't delivered on our design intent. Since some of our players have become such great rules lawyers, I am hesitant to mention a magic number at which point the ratios become unacceptable to us. Let’s just say you shouldn't often be in a situation where you pass on an item in a later tier of content and keep an item from an earlier tier solely because of the stat allocation. As an example, if haste were so much better than crit for an Enhance shaman that he kept his Blackwing Descent (first tier) gear and dismissed all of the Firelands (second tier) loot as garbage simply because it had a lot of crit, then we have a problem. (We're less offended if great trinkets or set bonuses sometimes outlive their tier.)
There are some exceptions. Hit and expertise have real caps so it's intended that they are very good up until their caps. It's a longer discussion than I can fit in this blog, but we like how cappable stats affect the gearing decision -- the puzzle of which gems to use or how to reforge is often played because you want to hit that hit number without going over. Likewise, Spirit is the kind of stat that healers will only need "enough" of, and once they feel comfortable with their mana regen, additional Spirit will be less attractive than other secondary stats. (This sort of thing also depends a lot on the encounter mind you.) Mastery is a new stat for us, and while it's working great for some talent specs, we'll almost certainly have to tweak it for others in a future patch (Unholy and Retribution come to mind, because their masteries currently affect sources of damage that just aren't large enough to be competitive).
One additional point worth mentioning: we don't design every item to be perfect. While we try to avoid actual booby prizes (I’m looking at you, Vendorstrike), we definitely go out of our way to make some items more highly optimized than others. If you knew every loot drop was certain to be an upgrade, then there isn't much reason to have stats on equipment at all -- you could just reduce everything to "power" that scales with item level. It's quite intentional for example that Ashkandi 2.0 has a very generous weapon speed.
Looking to the future (beyond Cataclysm), primary stats are the kind of thing we might mess with some more. We’d dearly love to be able to offer you the choice of Agility versus haste without the former being a no-brainer. We’d have to change a lot of the combat formulae (such as how much attack power you gain from Strength or Agi) to increase the relative attractiveness of the secondary stats. However, that kind of overhaul might be worth it in the name of offering more interesting itemization choices. Items are one of the very few rewards we have to offer max-level characters, and we constantly struggle to make sure they are in fact rewarding without becoming too predictable.
Greg "Ghostcrawler" Street is the Lead Systems Designer for World of Warcraft and knows how to tank the whale shark.