1) SQL Server 2008 R2 Reporting Services doesn’t support Windows 2003 (or R2) Itanium based servers
2) SQL-DMO support is stopped in SQL Server 2008 Express
3) Cannot enable Web Assistant with sp_configure. MS recommend using Reporting Services
4) Surface Area Configuration Tool is discontinued in SQL Server 2008.
5) Unattended Installations. SQL Server 2005 Scripts using /Q or /QS also need /IAcceptSQLServerLicenseTerms
6) SMO for 2005 is modified slightly for 2008. Applications using 2005 SMO dlls should refer to 2008 supplied dlls
7) Consider the deprecated list of features in 2008. Lists exists for features not appearing in the next version and features not appearing Review code and consider changing. Some examples are: DBCC DBREINDEX replaced by REBUILD option of ALTER INDEX , DBCC SHOWCONTIG replaced by sys.dm_db_index_physical_stats. Check the Microsoft site for the full list
8) SQL Server 2008 R2 Enterprise Edition uses 8 CPU as maximum . Datacenter is required if more than 8 CPU is required
Download a detailed document from Microsoft
See Also
SQL server 2008 R2 – Silent installation for the SSMS and client tools
