The biggest improvement is that Snow Leopard Server is now completely 64bit, meaning it can access up to 16TB of RAM, a theoretical amount of RAM, obviously. Overall server performance should benefit from this move to 64bit; especially virtualisation will really like the extra RAM.
Snow Leopard Server also comes with Podcast Producer 2, a new version of the tool which allows you to, well, create and publish podcasts; one of its new features is Podcast Composer, which allows you to automate the completion and publishing of podcasts. iCal Server has also been bumped to version 2, which includes, among other things, real-time calendar access from iPhones, and a new web based calendar client.
It also comes with Wiki Server 2, Address Book Server (which makes sharing contacts easier), and Mobile Access Server, which enables you to access firewall-protected services with your mobile Mac or iPhone. Oh, and in case you're wondering, ZFS support has been dropped completely from Snow Leopard Server.
Snow Leopard Server will be released alongside the client version in September, and it's Intel-only as well, obviously. Snow Leopard Server, with an unlimited amount of client licenses, will set you back 499 USD.