In Magic:the Gathering design, people often speak of three player archetypes: Timmy, Johnny, and Spike.
Rosewater writes that "Timmy likes to win big. He doesn’t want to eke out a last minute victory. Timmy wants to smash his opponents. He likes his cards to be impressive, and he enjoys playing big creatures and big spells." In WoW terms, we'd probably ascribe the "casual" label to Timmy.
"Spike is the competitive player. Spike plays to win. Spike enjoys winning. To accomplish this, Spike will play whatever the best deck is. Spike will copy decks off the Internet. Spike will borrow other players’ decks. To Spike, the thrill of Magic is the adrenalin rush of competition. Spike enjoys the stimulation of outplaying the opponent and the glory of victory." We'd definitely tag Spike with the "hardcore" label.
But there's a third archetype in Magic, one that may be missing from WoW: Johnny. "Johnny likes a challenge. Johnny enjoys winning with cards that no one else wants to use. He likes making decks that win in innovative ways. What sets Johnny apart from the other profiles is that Johnny enjoys deckbuilding as much as (or more than) he enjoys playing. Johnny loves the cool interactions of the cards. He loves combo decks. Johnny is happiest when he’s exploring uncharted territory."
I know that Johnny exists in WoW. He's the guy who is constantly tinkering with off-the-wall builds; who derides "cookie-cutter" builds on the forums; who attempts to challenge a paladin raider who declares (rightly) that you should use spell crit gems instead of +heal/+int gems. (This last one is from recent threads on the paladin forums. Osc is arguing with Aus and an whole lot of paladin raiders on this issue. Osc is wrong, but I understand why he is arguing.)
Yet Johnny occupies a very different place in the Magic hierarchy than in the WoW hierarchy. In Magic, Johnny is respected because occasionally his creations are tournament-level, and often end up being the most powerful decks and/or breaking the format. In WoW though, Johnny's creations are almost never as good as the cookie-cutter builds and options. Johnny is considered misguided at best, a bad player at worst, and generally to be wasting his time. The hardcore guilds are rarely tolerant of Johnny.
So why does this difference exist? I think it exists because the math behind WoW is a lot simpler than the math behind Magic. In particular, the costs in WoW (mana, rage, energy, time) are a heck of a lot simpler than the costs in Magic (tempo, card-advantage, mana-curve). The value of an ability rarely changes in a fight. Thus the best abilities and sequences are quickly calculated. There is no real room for Johnny to experiment.
I think that when designing a game, it's important to try and ensure a place for Johnny, to look at what you can do to prevent cookie-cutter builds/gear/rotations from dominating and stifling that segment of your playerbase.