First, let's begin with bad news: the developer preview's minimum system requirements. Lion demands at least a Core 2 Duo processor, which means Macs with Core Solo and Core Duo processors are out. These machines were sold as late as 2007, making this move rather... Unfortunate. It's clearly a case of Apple trying to get you to upgrade; Windows 7 runs just fine on machines over 8 years old (my media centre is a Pentium IV from 2002; it does full HD without a hitch), and I'm pretty sure Lion would run just fine on Core Duo machines as well (Core Solo might be problematic).
Another casualty is less dramatic, and that's the complete removal of Rosetta, the PowerPC emulation layer built into Mac OS X. You can use System Profiler to find out if you're still running PowerPC code, but my guess is you peobably don't, and on the odd chance you do, it's most likely an application you haven't updated in a while.
Mail.app has been overhauled, and finally gets the vertical preview pane layout I've been adding to Mail.app using a plugin for years now. People tend to call it the iPad mail layout, but that's incorrect; the vertical preview pane layout was first introduced (as a default at least) by Outlook 2003. I'm sure someone else actually was the first to get it, but of the major clients, Outlook 2003 was the first. It makes much more sense in this day and age of widescreen displays. Apple maintained the classic, horizontal preview pane as an option.
Did you know Windows Vista and up have file versioning built right into the file system and operating system? Just click on any file or folder, and select the previous versions tab. Big chance you had no idea this was in there (I'm not entirely sure if Home editions have it - I think it's Pro and up). Well, Apple is introducing the exact same feature in Lion, except here, it works like Time Machine does. While Time Machine is crude at an implementation-level, the interface is - while overly flashy - quite usable, and makes backing up something easy.
Versions is part of the new Auto Save feature. Lion saves your document every time you open it, as well as every 60 minutes. You can also click "Save as a version" to do so manually. All these saves can then be browsed per-file in a Time Machine-esque interface. Files can be locked, too, in case you don't want Auto Save to mess with your document. Whether this happens at the file system level, or requires Time Machine, I don't know.
It also ties in with the resume feature, which works on an operating system level as well as on the application level. Like applications on iOS that makes use of it, applications can save state in Lion. The operating system itself does this as well, so after applying updates, for instance, you can be back right where you left off. This feature has been available on other operating systems for a long time (on the OS level, at least).
Applications need to be specifically designed for Lion to make use of these features. In other words, we'll see support for it in Microsoft Office:Mac 2023 and iTunes will never get it.
Apple also added a recovery partition to its operating system, where it installs a number of tools so you can service your Mac in case Mac OS X decides to throw a fit. I'm not entirely sure if this includes the entire installation DVD (so you can perform re-installations), but it's a welcome, if somewhat dated and long-overdue, feature nonetheless.
Other gems that have been dug up are TRIM support for SSDs, overlay scrollbars (those things that disappear when you're not using them), and iOS-like scrolling. iOS-like scrolling feels like something that's going to cause considerable headaches at first: you'll now have to move your fingers down on the touchpad to scroll up, and move them up on the touchpad to scroll down. This is the exact opposite of how it is today on, well, any computer. This is going to be quite frustrating at first.
The Finder has also received an overhaul, but whether this is overhaul is skin-deep or if it will finally cure it of the severe case of split personality-disorder remains to be seen. It looks nice at least. Note that Apple has been sending takedown request for these videos, so it may cease to work shortly.
Also of note is that Mac OS X Server is no longer a separate product; it's all become part of Mac OS X client. This means you can set up any Mac you already own as a server, without having to buy a separate package.