ActiveSync is greatly enhanced, you can now sync between a mobile server, and another computer. So for example, I can now sync my corporate calendar with my personal home computer's Outlook calendar.

You can sync, media files via ActiveSync (real nice if you own a Media Center). In the past you had to have either MCE or Media Player open while the conversion and sync occured, now ActiveSync does it all.

Program installation is enhanced, now when extracting a cab file you can select where you want placed the install.

Phone is better, now you can go into your call history and add a call to your contacts. In addition you can add notes to calls.

Mobile Office - Better Word formatting support (tables), Excel (graphs), and now Mobile Powerpoint.

GPS port virtualization, you can have multiple GPS applications running all using the same GPS device (this is a guess, I haven't tested it out yet).

Better interface.

Longer battery life.

Interface improvements on contacts, calendar, and phone.

I'm sure there are more, and you can search the forums (not just the i730) about WM5 improvements.