Stephen Totilo over at MTV's Mutliplayer blog actually canceled his WoW subscription recently, and to his surprise, actually found the experience easy and fun. He hadn't updated the game since last September, and speaking as someone who's played the game many, many times since then, I can safely say he was wasting his money. A great MMO is worthless to you if you're not playing it.
Thinking back on it, I believe I've only canceled one MMO ever, and that was Dark Age of Camelot (and I only canceled it because I started playing WoW). It was so long ago that I don't remember if I had any problems or not, but it must have gone smoothly enough. Other than that, I've pretty much considered all my subscriptions money well spent, so I haven't had to end anything.
But surely there are many more cancellations going on. When have you stepped up and ended the subscription, and for what reasons? Ever had a bad cancellation, or a really good one? And maybe we can help, too-- if there are any subscriptions you're paying right now that you shouldn't be, feel free to commiserate about them here-- we'll help you quit, if you need it. There are always more MMOs in the sea, if we're allowed to mess up a metaphor.