Reading a book is fun. If you have to do a review on the book for the newspaper by Friday, it becomes work.
Writing code is fun. If you have a deadline next week, it becomes work.
Spending time with that special someone is fun. After tying the knot and having no other choice makes it work (or so I'm told).
Calculating sports match statistics is fun. Spending the same amount of time to balance your checkbooks is work.
Is commitment the difference between fun and work?
P.S. Yeah, I had a Godin moment.
Update : After reading all the interesting thoughts by you folks in the comments section, maybe spontaneity is one of the major differentiatiors?