Snow Leopard was a major code rewrite. While it looks just about the same as its predecessor, it takes up less disk space just because it's more efficient. Also, over the years, Apple has pulled a lot of stuff that used to be included in the OS installation. Instead, you're prompted to download the extra software if you use something that requires it. I don't remember which OSes removed which features, though. X11, which few people need, is gone. If you try to launch an X11 app, you'll be prompted to download the software to run it. And the OS used to include a boatload of printer stuff that's now online instead. Actually, with newer OSes (Mountain Lion and later, I believe), if you happen to buy an AirPrint compatible printer, you don't need software for it at all. It'll set itself up in about a minute. (If you have an iPhone or iPad, you really want AirPrint; the easy setup on a Mac is a bonus.)