The Administrative Differences
Sure we use Management Studio instead of Enterprise Manager, but that's not really a big deal.
Security is one area that is a very nice improvement. The separation of the schema from the owner makes administrative changes easier.
Another big security change is the ability to secure your web services using certificates instead of requiring authentication using a name and password. Add to that the capability to encrypt data, and manage the keys, can make a big difference in the overall security of your data.
There are also online indexes, online restores, and fast recovery in the Enterprise Edition that can help ensure that you take less downtime. Fast recovery especially can be an important feature, allowing the database to be accessed as the undo operations start.
With SQL Server 2000, you were limited to using 2GB of RAM and 4 CPUs on the Standard Edition. The number of CPUs hasn't changed, but you can now use as much RAM as the OS allows. There also is no limit to the database size, not that the 1,048,516 TB in SQL Server 2000.
Summary
Security
In 2000 Owner = Schema, hard to remove old users at times
In 2005, Schema is separate. Better granularity in easily controlling security. Logins can be authenticated by certificates
Encryption
In 2000, No options built in, expensive third party options with proprietary skills required to implement properly.
In 2005, Encryption and key management build in.
High Availability
In 2000, Clustering or Log Shipping require Enterprise Edition. Expensive hardware.
In 2005, Clustering, Database Mirroring or Log Shipping available in Standard Edition. Database Mirroring can use cheap hardware.
Scalability
2000, Limited to 2GB, 4CPUs in Standard Edition. Limited 64-bit support.
2005 4 CPU, no RAM limit in Standard Edition. More 64-bit options offer chances for consolidation.
