I wouldn't necessarily say it's a bad thing, but that's only because of how WoW is a relatively solid game with a sturday fanbase and they manage to appeal to all kinds of gamers; raiders, pvpers, casuals, and even kids to some extent. Aka, the short of it, is that the "monopoly" as it were is one that doesn't suck at least; back when RO was the lead after EQ it was mostly because that's all there was. WoW provides alot, both base and additional free full-blown content(Entire new areas, rehauls, etc) to all the groups to boot.

So yeah...it may not be the best thing that WoW is so tough to go up against, but at least it's more or less worthy of it's "Unbeatable" title.