It looks like to me the only major addition from Mac OS X 10.4 to Mac OS X 10.5 is Time Machine and Spaces (which you can other freeware version of this). These to me are the only notable upgrades from Tiger to Leopard in my opinion.
I have been around on these forums during the development of Panther, Tiger, and now Leopard. For those of you interested, there is a distinct pattern. Every time, without fail, a significant number of users will always come out suggesting that the upgrade is either overpriced, insignificant, or unnecessary. They also always have one thing in common: they are mistaken. If you had listened to all the complaining and opinions on this forum since then, you would have come to the conclusion that there really isn't any big difference between Jaguar and Tiger! Which is ludicrous! Here are some obvious examples of the macrumors disease of oversimplification:
Panther Upgrade: "I'm not paying $130 for just Exposé and user switching!"
Tiger Upgrade: "Dashboard and Spotlight are really the only new features!"
Leopard Upgrade: "Time Machine? Is that all they can come up with?"
Look, an operating system is a huge piece of software and technology. There is allot more to it than meets the eye. Most of you are utterly clueless about the inner workings of Tiger; and ironically, these invisible things you have taken for granted have probably affected you more than the 'front-stage' features (like dashboard) that you thought were the most important.
Here is a simple analogy I'd like you to try on for size. The Macintosh operating system is almost like a car. There is vast difference between a 1960 Cadillac with 300,000 miles on it and a 2007 Cadillac with 50 miles on it. And guess what, at the end of the day, both cars will get you from point A to point B! But it would be idiotic for me to assume that there aren't other drastic ramifications to consider between the two. You can't oversimplify the situation and say that the 1960 model "does the same exact thing" and that "you would be wasting your money on a new one." If that's your philosophy, I suggest you save a few bucks and look into a used copy of Puma on Ebay.*
So, if you are new to the Macintosh operating system, and this will be your first upgrade, I advise you against taking the impressions of every obsurd notions that Leopard is anything less than earth shakering; because it is!!!
* slight exaggeration for affect