What cannot be done in SQL Server 2008 Web Edition?
Have you seen this matrix as well?
http://msdn.microsoft.com/en-us/library/cc645993.aspx
From what I can tell Web excludes a lot of the more advanced features, especially those related to development and data mining, but nothing that would stop you from using it for a non-enterprise web site's data source. It seems to be between the Express version and Standard version.
One of the biggest limitations is that it can't be a publisher for SQL replication. If you want to have multiple database and replication data between them, then you need at least one SQL server to be Standard or Enterprise to act as the publisher.
http://www.microsoft.com/sqlserver/2008/en/us/editions-compare.aspx
Edit: Sorry, you've probably already looked at the above.