Another useful piece of information to add to this thread would be information about status EXPIRATION policies/tier drops on the various airlines:

For example, even if I make NW platinum this year (flying 75,000 EQM), if I fly almost no miles on NW the following year my status will drop to nothing. But I've heard that on some other airlines (I think AA) you only drop one elite level per year, so it gives you at least a couple more years of SOME status on that airline, which could be important to some of us.

Does anyone have a quick summary or link to various airline tier-dropping policies?