For developers, Leopard is great. It offers many new features, and most of them are not even mentioned by apple, as I constantly hear about Objective C 2.0. But there are many other things, too.
Generally, you can look at it this way:
Eventually, you will have to upgrade. Leopard will be more stable than Tiger, and applications will start taking advantage of QuartzGL (remember Quartz 2D extreme that was disabled? It's changed name, and now, although it is disabled by default, it is no longer buggy, and devs can use it to their programs) meaning greater speeds. Multithreaded OpenGL will also result in improved graphics performance in all graphics intensive applications.
- I like the new spotlight. I find it great. And the new spotlight that will find things in menus is also excellent. It is also much faster.
- Time machine. I intend to use it, someday, but to many people is life saving.
- Cleaner, more consistent UI. I like the new icons, and the new appearance.
- Finder is snappier, no matter what everyone else says.
- I like the new look in finder windows. Things are more organized.
- Memory management improvements.
- I really like the new safari. It's been stable for me.