– Protect authen info and/or data store?
– Many dangerous code bugs?
– Transfer data fastly?
– Daemon not crashed suddenly?
– Stored data integrity & consistency
– Easy to install & configure & operate?
– High CPU, RAM, IO?
* Design & feature
– Quota limitting per folder
– Bandwidth throttling per user
– IPv6(in future plan is also OK)
– Logging detailed & stored remotely
– Open source
– Active development
+++ Most popular guys recommended: vsftpd, ProFTPD.
And I will go to with vsftpd :).