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.
The biggest way this affects the game is in terms of game design. Blizzard is on record as designing the game for people to be able to come back and still be able to play it, even years later - the addition in Mists of Pandaria of a 'What's Changed' panel in the spellbook was part and parcel of the attempt to build support for returning players into the game itself. The level 90 boost was another idea aimed at the returning player who just wants to get into the game with friends. As a result, there's significant effort dedicated in the very design of the game to keeping it related to what has gone before. Thinking about this, we can immediately see how it works - the stock UI has changed minimally over the years in order to keep it familiar to returning players, as an example. Many elements of the game that have remained more or less the same have done so because changing them too much would create a bigger barrier to re-entry for players.
This design goal means that the game is constantly in tension between necessary change and a desire for continuity between where it is going and where it has been. The game changes - it's unavoidable, and the changes can become severe over time. If you played last in vanilla, you'd recognize the game when you logged in today, but actually playing it would place a host of unfamiliar things to you. Flexible raid scaling, the new talent system, two new classes, five new races. The dungeon and raid finders, scenarios, heroic dungeons, none of these existed in vanilla WoW.
To a degree, this pseudo-familiarity created by the effort to keep the game accessible to returning players is a good thing, but it also creates the false expectation of the game playing the same. Certain elements haven't changed, after all - the way combat presents itself isn't wildly different - but in practice whether our familiarity comes from having left and returned, or from having stayed and played the same thing for years, that familiarity can be misleading. The game has changed, and continues to change. You have to always be capable of forgetting the way things used to work if you intend to keep playing. This is only magnified further by switching roles or even classes.
There's actually a tremendous amount of unlearning needed to keep playing through one of these switches, or through the boundary period of an expansion change. We're going to see a host of changes to our classes in patch 6.0 - even if you play the same class, suddenly what they can do will be altered. (I still remember the period before Cataclysm where warrior DPS tripled and my undergeared night elf warrior was suddenly blowing up five mans.) There's a need for flexibility during any of these periods of transition. Remembering the past is great, but it doesn't always help you figure out what you're doing now.