Yeah I think personal greed takes over.  If everyone agrees and quits mining, and one person continues, he is one lucky bastard the next day because he just got a lot of bitcoins in one day, lol.

As for transactions I bet with enough support you could have a bunch of people quit for a "green" day.  Or if you found out that satoshi was, in fact, hit by a bus you might get a day of reverence.  But not everybody!