If I had to give an overall "feel" for Leopard Server, I'd say that in many ways, it's a shot across the bow of Microsoft's dominance in the small to medium business (SMB) market. The new features, while quite useful in Apple's traditional market, are even more useful for businesses tired of paying the never-ending chain of CAL taxes that Microsoft vacuums out your pocket.
One of the biggest annoyances with OS X Server has been just keeping an eye on it. While Apple does give you the Server Monitor utility that tracks hardware issues, and you can use Server Admin for service monitoring, neither of those are great for "quick glance" monitoring. With Leopard Server, Apple is shipping a Dashboard Widget that will give you, well, a Dashboard view of the services on a given server. Nothing major here, but a nice way to make a sysadmin's life a little easier.
Another feature in the "make the sysadmin smile" set is more automated setup of clients. With the new Directory Utility (replacing Mac OS X 10.4's Directory Access), Apple is making it easier to set up applications like Mail, iCal, and iChat in a fairly automatic way. With a little luck, there will be an API for this, so third party applications can take advantage of it. However, the fact that Apple has listened to the number of complaints about the effort required to set up a client out of the box is a good sign, third party access or no.
Without going into a huge amount of detail, CalDAV uses existing standards like IMAP, WebDAV, and iCal, and adds in features like Free/Busy, Room reservations, etc. This allows CalDAV to be fairly lightweight, but get a lot of work done. While a lot of free calendar clients, including (obviously) the Leopard version of iCal will work out of the box with a CalDAV server, the elephant in the room is Microsoft, specifically the Outlook and Entourage products which, contrary to popular thought, are not hated by all their users.
Currently, there are some MAPI-based connectors in various stages of work for Outlook, and there's some talk about Outlook 2007 playing well with CalDAV as well. However, there's nothing I could see in the way of an Entourage connector, and all the Outlook Connectors are using MAPI. That's a shame, because if someone were to write a connector that used Outlook Web Access (OWA) for the connection mechanism, then they could support both Outlook and Entourage for the same amount of work. Evolution can use OWA as well, so that's a third bonus.
If Apple were to provide such a connector, it would be marketing genius, as they could go into smaller Exchange shops, say a thousand users or under (and with the Xsan-based clustering features in the Calendar Server, a thousand users or less is quite doable), and say "We can replace Exchange/IIS in your company, your Mac/Windows/Linux users will never notice, and you'll never have to pay CAL costs again." As long as the company wasn't getting into some of the Exchange specific stuff, like workflow, it would make Leopard Server into a VERY viable replacement for Microsoft SBS.
However, Apple is taking collaboration further in Leopard. It's not enough to just schedule meetings, you also need an easy way to work on projects once the meetings are done. To that end, Apple is implementing a Wiki Server in Leopard as well. There's no need for me to point out what you can do with a Wiki -- Wikipedia is the best evidence of that you can find. But I will say that Apple is putting a lot of thought into making their Wiki almost stupidly easy to use. That's just as important as the Wiki feature itself, as a tool that's too hard to use is essentially useless.
Like Leopard, Leopard Server is 64-bit from top to bottom. While the value of 64-bit isn't immediately obvious in a client, its benefit is clear on a server. Databases, large scale video serving, large scale file serving, and large scale collaboration immediately come to mind as services that would benefit from 64-bit memory and disk access. Servers always need more resources faster than clients do, so the overall 64-bit support in Leopard means that it will take a little longer to hit a wall with regards to memory and disk addressing.
Other new features include iChat Server improvements like chat logging (for regulatory compliance), better federation features (to make linking with other chat services easier), Kerberos integration, and server to server encryption. Open Directory 4 adds an integrated RADIUS server, something that anyone with a wireless network will appreciate. Apache 2 finally becomes a full citizen in Leopard server, Apple is adding Ruby on Rails, and Kerberized NFSv3 as well.
No, this is not a really detailed look, as most of the details are still under NDA. But even allowing for that, it's pretty obvious that with a little more work, Leopard Server could really help Apple make some serious market share gains in the SMB market, and they won't have to become a Microsoft or an HP to do it. All in all, Leopard Server is going to be very interesting as it moves toward the spring 2007 release time.