SQL Server 2000
Security:Owner = Schema, hard to remove old users at times
Encryption:No options built in, expensive third party options with proprietary skills required to implement properly.
SQL Server 2005
Security Schema is separate. Better granularity in easily controlling security. Logins can be authenticated by certificates.
Encryption: Encryption and key management build in.
These are two major things
ref: http://wiki.answers.com/Q/Difference_between_SQL_Server_2000_2005%5B/url%5D