Just been having a short discussion with @brianwaltersPMI on twitter about the fact that OS X Lion doesn't ship with Adobe's Flash Player by default. I think this is a great decision on Apple's part and i'll go some way to explain why they've done it.
1. Security
Everyone on Windows know's how often Adobe update the flash player with security fixes. My fiancee tuts every time the popup on windows appears demanding she update her plugin, and i just laugh at it.
Flash doesn't come on a Mac by default because people years on from now will install and run old, insecure versions of Flash, simply because they're none-the-wiser. The Flash update system / procedure on a Mac is far less obtrusive to the point that you never really know if you're up-to-date.
By not shipping Flash, they're making sure that those that do need to use Flash, will start with what ever the most recent version is at that time, leaving it to Adobe to make sure that updates happen.
2. Browser capabilities
Flash isn't the only technology out there that can do swishy effects, and Apple is making a point. Apple supports the progress of HTML5, the next flavour of the language that is key to every site on the web. HTML5 brings effects and transitions that would have normally been done in Flash (or indeed javascript). HTML5 intends to be entirely browser driven, meaning that you will not have to install a "plugin" such as Flash to run it.
HTML5 won't remove Flash from the web altogether but it will make it less popular across the major sites that people use. As soon as all the browsers have a consistent level of support of HTML5, sites across the web will transition over to HTML5, from Flash or Javascript. I'm confident of that.
3. Efficiency
Most die-hard mac users will have noticed that when you browse a site that uses flash heavily (or not in some cases) you're Mac's go nuts. My quad-core iMac tries to cool itself as quick as possible when on a Flash site, as it's incredibly processor and memory intensive. In other words, it's burning a helluva lot more fossil fuels to view a Flash based site.
Apple are very keen about their laptops and mobile devices and with more features comes more power demands. By not using Flash on devices it knows that it's not going to end up draining their batteries in 5 minutes flat just to watch a cat going nuts about an Apple on a bed on youTube.