The thief does not even need to "crack" your password. Apple was nice enough to leave a password reset utility right on the recovery partition to make it easy for the thief.
Just command-r boot to recovery and start Terminal then select the admin account (yours) and type in "resetpassword" then enter a new password. Now reboot and enter that new password. Presto.... the thief has full access to your account/data.
Like others mentioned, most thieves will just wipe the drive and sell the machine, but if for some reason they want access to your data, it is very very easy to do.
Maybe use the built in OS X Filevault encryption on your next machine. That is very secure and the PW cannot be reset like I described.