[BlueNinjaGo]

Well we too try to warn our users ahead of time about any changes. If it's a feature we're adding for the users, we might make a poll asking them if they want and/or would use it.

There are times that we add features or make changes because we want to. If they choose to complain, we tell them it's our choice in the end and we aren't changing it. If they continue then this is what I do:

I made a joinable usergroup called "Closed". (Basically a banned usergroup with the ability to view forums, not post or anything)
I made a thread and said I don't want anyone to leave, but if you wish to, join the usergroup.

Anytime someone threatens to leave the site (a.k.a. good-bye threads) or they complain too much I just post a link to that thread. They get the message.