I am in a similar situation, although much less extreme.
I've had some issues with game compulsion (thankfully they haven't harmed me as much) and I've also faced that ethical dilemma when I decided to get into indie development (as a hobby).
If you're willing to enter the industry of MMO development, I think you should forget about those "ethical" design principles right away. Most MMO are designed with operant conditioning in mind; they're glorified Skinner's boxes. Even when a feature appears designed to enforce casual play (think WoW's "rested" system) it is often implemented with the idea of giving hesitant players an incentive to log in. "I don't really feel like playing, but I should have a full rested bar by now so hey".
Not even EvE online, which has been jokingly referred to as a game that doesn't want you to play it, is free from this sort of stuff. Okay, mechanically it may not necessarily reward obsessive players, but it is built with social interaction in mind, and you will eventually get to a point in which your corporation requires your presence.
If you want to develop in an ethical way, I think MMOs are a complex issue. It is unlikely you'll get to lead the design of a MMO without doing a lot of grunt work first, and even if you do, it's doubly unlikely any publisher or investor will back a risky enjoy-but-don't-get-addicted scheme. However, there are many other genres that are focused on giving the player a great experience, without attempting to condition him or her to become a slave.
That said, some people will become addicted to anything, and you shouldn't let that reality haunt you. Just think of the quality of the experience. What makes it compelling? If it is addictive because it is a blast to play, rejoice. If it is addictive because it deliberately exploits the way your brain reacts to short term rewards, social pressure... Then you have a problem.
Thank you guys, really really awesome feedback here!
@sunandshadow: Great list of taboos! Some may be a bit out of the scope of the project, but others are spot on. In fact, I have to thank you for broadening the idea of taboos, I had only thought of them as conversational taboos, as in things you can't really talk about (such as sex in a really conservative Whisperer village). Your ideas really expand on that, and I particularly like stuff like the seasonal one (the game will involve deity festivities and events that might impose temporary taboos like that).
@Orymus: I certainly want to convey the image that Shining People are thoughtful, but it wasn't my intention to show them emotionless. In fact, the Whisperers would be closer to the Vulcan idea of emotions than them. I picture the Shining People like less evil Dark Elves from R.A. Salvatore novels, intellectual but also very prone to passion and emotion (often negative). In contrast, the Whisperers behave more like Japanese society; polite, reserved and calm.
You're right that in case of The Shining People it may be hard to put together these two concepts. I think it works best if you picture them in a frame of moral ambiguity (not sure if I'm making sense there) they're highly ambitious, and often see themselves above good and evil, so if killing an "inferior" foreigner is needed to protect their people, they will not hesitate to do it.