I went to a bunch of SQL Server 2008 talks in PASS 2008, the only 'killer feature' from my point of view is extended events.
There are lots of great improvements, but that was the only one that got close to being a game changer for me. Table value parameters and merge were probably my next favourite. Day-to-day, IntelliSense is a huge win.. But this isn't really specific to SQL Server 2008, just the SQL Server 2008 toolset (other tools can give you similar IntelliSense against SQL Server 2005, 2000, etc.).
