There would have to be a lot of added content to justify a full blown retail xp. Like the new XP for WoW is adding two new races, and with the amount of equipment in the game thats a pretty big undertaking. Each of those races gets a capital city, which are pretty big in WoW, they're adding new instance missions, new enemies, new equipment, new quests, new professions, new areas... their regular patch updates I think pretty much just added a new area and "fixed" some balance issues.
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So where's this new content on a regular basis? You mean, balance fixing? <_<