The term power leveller has nothing to do with workers in gaming sweatshops; it dates back to the days of muds on BBS, when many of them were only available for a few hours a week. Some players began levelling characters as quickly as possible during the time available, and spawned the term. At the time, power levellers were more often than not the players who understood the various rule sets inside out and were often themselves gm's(had a different meaning back then) and were actually looked up to in some circles, due to their dedication (don't take this out of context, they were only able play a few hours at most).
The term has only recently (last 4, 5 years) been linked to individuals working for sweatshops, when they began marketing themselves as power levellers and offering it as a service. Sadly, because of that, the original meaning is now used as a term of derision by newer members of the mmo scene who do not know better. At one time it was a compliment to be considered a power leveller, now half-wits throw it about by way of insult; how quickly times change.
That is why I think of a power leveler know as a gamer who knows the ins and outs of leveling fast in an MMORPG. Just like ElitistjJerks.com was supposedlly created by lawyers who wanted to get the most out of their play time in WoW so they started coming up with ways to better do things in the game. They I also think of as power levelers. Even though that site has taken on a different mind of its own now it seems.