In the context of an MMO, it would probably be useful to distinguish control from understanding.
One player might acquire mastery at moving their little avatar around effectively and firing skills quickly, but may only have an average understanding of the combat related systems and statistics churning away behind the scenes. Another player may understand the game’s systems inside out, but may be slow and clumsy with the controls.
In both areas, I’d say that there seems to be a clear path from novice to mastery, and all players will reach different points along those respective paths. Also, since most MMOs tend to be Diku-alikes and therefore quite similar, a level of mastery in one tends to transfer readily to others.
The problem with most MMOs is that your avatar’s relative power is determined more by statistics than by player skill, and since the improving of stats tends to be predominantly a function of time invested, ‘mastery’ is very difficult to determine, unlike in say, Chess.
Steve may have shied away from EVE, but I’m going with it.
In the grand scheme of things, EVE is fairly simple. Skills always train, finding information is pretty easy, and getting a grasp of basic concepts is fairly well explained.
However, there are many deeper topics which aren’t as simple. Fleets, PvP Combat, how standings work, soloing harder missions, EWAR and the like are all fairly complex topics which take a good deal of time to truly master.