I was able to have some hands on experiance on SQL Server 2005 express edition today.
Some of the obvious changes are really good to mention:
Schema is playing a major role in this edition. All these years it was like a simple equation where user=schema. This was giving a lot of problems to users who are not the owers. Some people even questioned the validity of schema and asked "do we really need schema with Yukon? (earlier name for SQL server 2005)"
Micorsoft has answered this question well. All the system tables are located unser "sys" schema. Schema is a way of classifying the objects where some pre-defined schemas are available.
Another change is the enhancement to TSQL. It supports CTE (Common Table Expression) which reduces the effort on query development. However, We need to check on the execution plan to see how it is actually implemented.