World of Warcraft constantly changes. It changes in patches in small ways (buffs change, spell power changes making a formerly useless ability stronger, or a strong ability weaker) and it changes between expansions in much larger ways. If I still played a fury warrior today the way I would have played one back in Wrath I'd be using Whirlwind as part of my rotation.
Between player action (players roll alts, switch mains and change roles) and the game's inexorable forward progress, there are always new things to learn, which require us to unlearn what we did before. Factor in returning players who take weeks, months, even years off - I've seen a lot of Hey, I left the game in X expansion, what's different now emails in my time at WoW Insider - and you have a continuous problem for World of Warcraft in people who have, in essence, a different game in mind when they play. This issue affects gameplay in numerous ways, both for those players (and eventually, we're all those players) and for the game itself.