This question may rise every time new version comes into the market. People will buy new version if it is going to make their life easy and this is one of the common interview question as well if you wrote in your CV that you know MS-SQL Server 2005 and 2008.
Basically one can write a book on the differences as Microsoft SQL Server 2008 comes up with lots of new features but we will look at it at glance.
Microsoft has divided new features of SQL-Server 2008 in four parts.
Enterprise Data Platform
You can dig details about sub-category of all above four categories from Microsoft Website. URL of the same is given at the end of this article. I will explain some of the features which I like most.
New Date and Time Data Types: This feature gave solution to age old problem of Microsoft SQL Server. You can now have only Date data type without any time and you can have only Time data type without having any date stamp. You can have precision up to 100 nanoseconds for Time data type. One more advancement is you will have Date Time Offset which will store UTC time zone aware value.
Table Value Parameter: We had not any way to pass complete table to Stored Procedure in any of the previous version of SQL Server so we have worked with workaround solutions but now we have a way to pass complete table to stored procedure.
Spatial Data: this is really amazing data type. It allows storing Latitude, Longitude and GPS based data entry to be stored.
Encryption: This feature allows encrypting the database backup so that it will prevent from the data tempering.
Office 2007: You can directly export your report from SQL Server Reporting Service (SSRS) to word file and you can use your word and excel file as a source of report in SSRS.
Merge: This is also one of the beautiful features. You can now merge UPDATE, INSERT and DELETE statement with SELECT based on the condition.
Reference: Ritesh Shah