If you boot into a 64bit kernel, none of your 32bit device drivers will work, so people who are upgrading will struggle. Drivers built for snow leopard will have both 32 & 64 bit code, but until that happens, booting into 64bit by default isn’t the right way to go.

That said, ‘booting into 64bit’ only refers to the kernel. All of the other libraries that apps depend on, AppKit, FoundationKit, libc, etc are all 64 bit, and will load if a 64 bit processor is accessible. Also, any program that was compiled with 64bit code will run in 64bits period, giving you that (2x) improvement.

The biggest problem for users will be that their software (Photoshop, Logic, etc) will be 64 bit, but their plugins might not be. If any program has plugins or extensions that are not 64 bit, it will have to be relaunched into 32bit mode to use them. Contrary to contentcreator’s comments, OSX does not ‘grind a bit’ when loading 64 bit code, nor does it load BOTH 32 and 64 bit. It loads 64bit code if it’s dependent libraries and the processor support it. If not, it degrades gracefully (and speedily) to 32bit code.

That said, nearly all programs 32/64/ppc continue to run well on SL, and all of them get to take advantage of GCD when they run, so they run a bit faster as well.

In the pre-release versions of SL, all of the Macbook Pros, Mac Pros and XServes could be set to boot in 64bit by default (without 32bit device drivers), and the others (with the exception of the MBA and Mini) could be booted with the ‘64′ keys held down. I guess they removed it to keep users from getting confused if their devices stopped working. My guess it will be the default for all machines once the driver community has moved all of their code over.

Finally, a new Contact Sheet view lets you see all of your pages in thumbnail view at the same time, a nice convenience. To get this view, however, you need to set your General preferences to Open All Files In One Window or Open Groups Of Files In The Same Window.

Jason Snell says:

Mentioning Microsoft in this case is notable because, as anyone who has ever tried to buy and install a Windows upgrade package knows, it’s a complete disaster.

You have to have the previous version of the OS available — and heaven help you if it’s not the exact right SKU — in order to install the upgrade.

If anything isn’t quite right, it all blows up in your face.

And of course you need to make sure you’ve got not just a serial number, but the right kind of serial number for the exact SKUs you’re upgrading over and with.

In contrast: Apple. No serial numbers, full installer, no need to go back to previous versions of the OS in order to install an upgrade.

Fans of the yellow button, fear not: by default, Snow Leopard still minimizes windows the same stupid way Mac OS X has for the last ten years. For us complainers, though, there’s a new alternative: A Minimize Windows Into Application Icon checkbox in System Preferences’ Dock pane. With that box checked, when you click on that yellow button, your window will still fly away into the Dock. But instead of disappearing into the mess on the right, it will minimize into the icon of the application it belongs to. That makes it easy to bring the window back. (Minimized windows are indicated in most programs by a diamond in an app’s Window menu; you can see that same list by Control-clicking on the app’s icon in the Dock.) Even better, this feature works with Exposé: When you invoke Exposé, all minimized windows line up together at the very bottom of the screen.