PvP combat, for example, is highly skilled in that there are thousands of potential combinations between spells, tactics, items, and talent distribution. Players can and have beaten others several levels higher, myself included, by using effective combat strategies targeted to my character's class.
The author downplays grouping, but effective group membership is an oft-overlooked skill that is essential for effective players to acquire. Tanking, pulling, healing, and crowd control are tactical skills that do vary in quality from player to player, and groups will suffer or succeed as a result. They aren't quantified as some in-game skill value obviously, but they exist nontheless. Not to mention effective leadership, marketing and recruitment (guilds, groups, raids), and social norms within guilds and parties (the WoW demographic is noticably lacking on understanding social norms).
Another important area not mentioned is the economy, which offers a significant amount of market-oriented information that requires skill, and ultimately this area is highly applicable to the real world. Competitive pricing, value-added production, supply and demand, speculation, market monopoly, bidding strategies, Auctioneer baiting, etc. - this is an extremely rich area.
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