Things that stand out:
Scripting
The scripting bridge in Leopard is extended from AppleScript to Ruby, Python and Objective-C. This means heaps more support for building apps using all of these languages. And the new object model means scripts are much more portable!
Boot Camp
Allows you to not only run Windows XP or Vista but “easily delete Windows”.
Dictionary
Includes Wikipedia. Nuff said.
Finder
Being able to share any folder is a welcome return (missing since Mac OS 9) but that’s not even “impressive”.
Screen sharing. Oh My. It’s the mutts nuts.
iChat
Again, screen sharing and iChat theater. They make NetMeeting look like your Grandad’s screen sharing application. And you can record these iChat transmissions so you can effectively generate interactive screencasts. Name watch for group chats makes this an excellent collaboration tool.
And you can make yourself invisible. Useful for avoiding some people.
Mail
First thing would be you can now forward messages as attachments rather than inline (which would be good for the people converting from Outlook). They’ve revived Data Detectors so Mail will be better at spotting appointments and addresses. Long way to go before it beats Newton Assist but still nice.
Networking
The new Airport menu. It identifies secured networks. So you can…uh…notice them…
Preview
It gets a lot of work, including some non-obvious stuff, like being able to pull GPS data from embedded metadata in an image. You can also re-order the pages in a PDF and remove Alpha backgrounds.
Printing
Leopard is meant to be smarter at realising which network you’re on and configuring your default printer. Drivers get a big update and, on top of that, Apple bought CUPS (Common UNIX Printing System) which means printing is going to get better and better.
Security
Digitally signed apps. Sandboxing. ACLs. Multiple User Certificates. Stronger Disk Image Encryption. Woot!
System
Live Partition Resizing. About time. Scroll any open window, even if not i the foreground. Choose which partitions of a shared volume to eject.
Terminal
It’s all growed up. Including saved workspaces and tabbed windows
TextEdit
Opens Word 2007 and OpenDocument formats.
UNIX
Now a Certified UNIX © and the file system is allegedly mult-threaded to allow multiple mounts to mount and unmount. Rails, mongrel and Capistrano are built in. Better support for BSD flat files. And say goodbye to NetInfo completely.
Should you upgrade?
What sort of stupid question is that?????