Brad DeLong offers a list:
In my belief that central banks had the tools, the skill, and the political will to stabilize economies at high levels of employment and low levels of inflation, and thus that fiscal policy and financial institutions policy no longer had any compelling stabilization policy role to play.
In my belief that large, leveraged financial institutions had sufficient caution and sufficient control over their derivatives books that their derivative positions did not pose major systemic risk.
In my belief that the principal threat to the world economy would come from the fact that in a crisis the shaky long-term finances of the U.S. social insurance state might provoke a collapse of confidence in the long-term value of the dollar.
I shared in one and two, though not three. I'm starting to believe in #3 however.
(That said, I would word #1 differently; for instance, I have long believed in automatic stabilizers and still do and I remain more skeptical of "ramp-up" spending than Brad. I would phrase #2 to focus on the balance sheet more generally and not derivatives per se.)
I also take the data on slow median income growth more seriously than I used to. I no longer think those numbers are a mere statistical artifact.
What can you all cite as changed beliefs? Examples like "Person X or Policy X turned out to be even worse than I had thought" do not count.
Addendum: Megan McArdle adds her list, mine could be longer too!