It's a fundamental problem in this sort of game design, unfortunately. When a game system is set up to be released in an episodic manner, or updated periodically, you face two related problems, which fall under the rubric of "ramping up" power. To some extent, this is due to one particular problem, the urge to introduce something slightly *or more than slightly* more powerful, just because that sort of thing is flashy and neat. That's what generally happens with CCG's like Magic: The Gathering, and systems like D&D, or Palladium. Computer games offer a second problem to that. They offer a lot more capabilities, in most ways, than tabletop systems and other game types, but they suffer from being essentially static. The players can't create their own content, really. The developers can add content, but that ultimately just makes the existing world a bit bigger, it doesn't create whole new worlds. That drastically limits the ability to keep paying players. The only other really strong alternative is to give the characters a sense of advancement, of getting better. And that invariably means a character is getting more powerful.
The need to do that is made even stronger, incidentally, by one of the tactics to make new world content hold attention longer, by making them difficult. Designers don't want players to just blow through new content quickly *they'll promptly start clamoring for more* so new areas often take forms like AQ40 or Naxxramas, which players are going to have to spend a lot of time on to thoroughly explore. You don't want the players to get too frustrated with the new content and leave the game for that reason, though, so you have to A) find ways to make progress "easier" *like getting new, powerful items as they go* and B) make the reward for going through all that frustrating content worth it. So the more difficult you make things, the better of a reward you have to offer in comparison to the power level prior to the new content.