It's fairly simple, I don't believe that Microsoft will spend five plus years on an OS that won't be better than XP. With most of the delay in release dates attributed to security improvements, I expect the security of Vista to be considerably better than that of XP. With features such as User Account Control (UAC) and BitLocker encryption, I'm expecting an OS that is considerably more secure.
There has also been mention of a new file handling system being introduced to Vista. This feature indexes the files on ones harddrive, making it easier and faster to search and access. I am expecting this to be a reality.
Introduction of AERO gives Vista a stunning new look that's supposed to make using Vista a much more pleasant experience. I expect this new interface to not only be stunning and attractive, but also to be unobtrusive.
I'm expecting networking in Vista to be safer with the introduction of Network Access Protection. Connections to Server 2003 and Windows Longhorn to be easier and more secure. Vista has a file cache system that caches files when working with Longhorn. I'm expecting this to be a reality and not have my work lost when a connection with the Longhorn server is severed during operations.