When an MMORPG has large experience point requirements for each level (usually to keep players playing and paying), playing is said to feel like running on a treadmill and the time spent is seen as a "leveling treadmill". You spend a lot of time killing before you see any progress in levels.
World of Warcraft has less of of a treadmill feel than a lot of other MMORPGs, though it is still there since characters can take a few weeks to a few months to level depending on how long the person plays and how much the person uses power leveling strategies.
See also Grinding.