I've signed up for the Windows Phone marketplace and submitted my apps. They're selling well - but I'm not actually planning on doing any more development, so I don't think I need to renew my subscription. What happens if I don't pay the Marketplace renewal fee at the end of the year?
You'll get a couple of warning emails at the end of the year, but after those if you choose not to renew your subscription your apps will be removed from the Marketplace.
Will people still be able to use my apps?
Sure. Microsoft won't revoke the apps from people's phones, but they will remove it from the Marketplace - so no one new will be able to purchase the app (and, obviously, no more revenue for you if it was a paid app).