Apart from an apparent lack of vulns...also choose a ftp server that doesn't need root prievlige to run (installing it will require root prievlige but ftp servers can *drop* their prievlige). Also choose a FTP server that can be run within a chroot jail environment..u'll find shell scripts that enable u to run ftp servers in a chroot environment. Running the ftpserver in a chroot environment will make things harder for the attacker even if he does manage to exploit the server even after a new exploit comes up. Of course nothing is fool proof...but still no harm in trying.
Used to use proftpd now I'm using pure-ftpd it's supposed to be faster (taking less memory) and supposed to have about zero exploit (that's just what I heard)