Another "Yes, but ..." here: As annoying as the shape-shifting Dock is, the addition of stacks is both useful and visually impressive. They let you see what's in a folder in the Dock without having to actually open the folder or even find it in your system.
Just click once on the folder, and icons representing all of your files sweep out in an arc across your screen. Click the file to open it, or click the arrow to immediately go to it in the Finder.
For Apple's Media Center fans, Front Row was a hit from Day One. It provided a great interface for browsing the iTunes media library and operated from a minimalist remote control that had a similar form factor as the original iPod shuffle. However, its inability to play non-iTunes Media was widely criticized; also, it only was installed only on new machines and ran a bit slow.
Leopard changes all of that. Any machine that runs Leopard now runs Front Row -- it's right there in the Applications folder. You can control it with the keyboard, a Bluetooth remote or the traditional Apple Remote.
It now also boasts the improved AppleTV Interface that allows you to browse your whole machine. With the proper codecs, it can play a much wider array of movies as well -- and not just from the host machine, but also from other machines and media servers on your network.
For many file types, the Finder has provided some preview capabilities for a while in column view, but Quick Look makes virtually every file preview-able. For Microsoft Office files especially, it makes it possible to just quickly skim a file for specific pieces of information without waiting for any of the Office apps to actually launch.
It's also great for getting a quick preview of attachments from within Mail rather than having to open or save the file first. All in all, it's a cool feature that turns out to be quite a timesaver in any number of situations.
This is an unsung hero of Leopard, a feature you're unlikely to notice until you stumble across it while using Mail, for example. The technology, first introduced in 1998 but dropped when Apple revamped its operating system three years later, can discern e-mail addresses, URLs, phone numbers and appointments in an e-mail. When your cursor moves over the text, Mail automatically places a dotted-line box around the word with an arrow allowing you to call up a contextual menu.
If the e-mail says, for example, "meet me tomorrow," placing the cursor over the word "tomorrow" calls up a variety of options when you right-click. Among these are Create New iCal Event; Show This Date in iCal; Look Up in Dictionary; or New To Do. E-mail addresses are recognized and can be added to your Address Book. Names can be opened in the Address Book, too. It's a little-touted feature that, once you get accustomed to it, you'll be using all the time.
Mail can detect a date in an e-mail message and quickly make a To Do out of it
Apple's data detectors can recognize a date in an e-mail message and put it on your iCal calendar.