The SQL Server Customer Advisory Team has a great blog and they have a good article here about upgrading to 2008. Lot of reasons to upgrade to 2008 (especially if you are on SQL Server 2000 or earlier). Check it out; http://blogs.msdn.com/sqlcat/archive/2008/12/08/ultimate-guide-to-upgrading-to-sql-server-2008.aspx
There are many reasons actually for upgrading.
My reasons are :
FileStream, Bakcup compression, and especially change data capture
I required this feature a lot in an application.
So if I was working with the new version, things will be far more easier