I am currently evaluating 2K8 and also trying to find the ultimate answer for this question. these are my thoughts:

+ Data compression. I think this is the most important reason to upgrade. We're running Axapta on a fast growing database with a lot of historical data. This is ideal for data compression

+ Backup compression. This is already the standard in third party backup tools. At last, it's in SQL Server

+ Mirroring. I like the feature that a corrupted database can be recovered by the mirrored database

+/-New datatypes. Nice, but as long as these are not implemented in Applications this is "useless"

on SSAS/SSRS I'll have to discover the new features.