While there are some serious improvements in speed and general functionality, the biggest thing I see with 7.0 is an ease of use issue. 7.0 is simply much easier to manage than 6.5 was. In addition, there is better ANSI SQL compliance, replication that really works, and a security model that is much more usable in large network environments.
Access is a really nice and easy to use desktop database.
From the standpoint of using it as a backend. That's an easy one - breadth from a developer perspective. There are a ton of DBA oriented books our there. There are even a few T-SQL books that have made an appearance recently. What Professional SQL Server 7.0 Programming has to offer that the other books don't is a clear progression for developers.