* Bitcoin should be regulated
* Bitcoin needs to have blacklists
* I'm safely storing my coins on
* "We'll never see $3 again" (when it was under $3), but you can replace it with $1, $10, $33, $266, $1200.
* Bitcoin is a Ponzi scheme
* Bitcoin isn't backed by governments so it will fail.  (Gold and silver aren't either).
* "Incoming crash imminent"
* "People can just print more bitcoins, they are on the computer"
And my favorite for now:
* "You can safely use tx id's to verify if a transaction has made it into a block" - MtGox.

There are so many though, it is hard to think of them all...