United's MileagePlus Program

One of the reasons that I prefer flying United (besides that I have generally had good experiences with them, unlike Northwest) is that I had miles building up in their Mileage Plus program.  In fact, I had 70,000 miles built up earlier this year...and they just expired them.  I didn't realize that they were going to expire suddenly.  Had I realized that, I might have used them for travel somewhere...if I could ever figure out the complexity of their mileage program.