The biggest has to be the automatic backup off all the connected machines on your LAN (well, up to 10 machines per the license). This happens automatically and is "smart" enough not to duplicate files. So the first backup of a XP machine might take 10gb, while backing up a 2nd XP machine might only take 5gb because it doesn't duplicate all the OS files. Yet you can recover from a complete client failure very easily.
The second thing I really like it is the shared folders. These are folders on the WHS (default includes folders like Software, Pictures, Videos, Music, etc - but you can add more folders easily) that are shared among all client machines. While this is possible without WHS, I personally always had issues with security, especially when trying to access from different OS systems (Vista vs XP, etc). Sometimes I'd have to input my user name & password to gain access - or they shares sometimes simply didn't show up, etc. Or I would try to modify a file while using my Vista machine and would be told I didn't have the correct rights to do it, etc, etc, etc. With WHS even an idiot like me can see & access (and modify) the shared folders - everytime - on any machine - without any issues.
The shared folders can also be set up to be duplicated. WHS will store copies on the folders and contents of said folder on two different disk drives. This adds another layer of safety and should allow you to recover data even if a drive is lost.
To summarize - the automatic backups and ease of folder sharing among all users is what really makes WHS stand out IMHO. I use other features too, but those are game changers for me at least.
