I think likely it would be a slow death. I'd log in less and less, until I found that I wouldn't log in for months at a time. At that point I'd probably be sensible and cancel my account -- although it took an inordinate amount of time for me to cancel my EQII account where I did more or less the same thing. I'd like to think this is how it'd go at least, I wouldn't want to quit in a huff over some silly bug or class change.
Of course, this is WoW, and I find myself uniquely invested in it. I've got characters with hundreds of days worth of /played on them, and I don't know yet if I could ever stop myself from being able to have immediate access to them. They are a hobby, and investment, just like a detailed model airplane or schooner. Then there's also what I do here, which gives me a little unique reason not to ever unsubscribe.
But alas, it might happen one day. And that's the topic of today's breakfast nook discussion. When will you quit playing WoW? How will you go out? With a bang or a whimper?