Regardless of how many Mac OS upgrades I've done over the years, there are always one or two drivers, application enhancers, or full applications that just don't run properly on the newly installed OS. With Snow Leopard, these situations should be rare as many developers have already done compatibility checking and worked out the bugs. Before you slip that Snow Leopard DVD into your Mac tomorrow and start doing the upgrade, there are a few things you may want to consider.
If you can tell it's Intel, it's swell!
It's important to remember that Snow Leopard will only run on Intel Macs. Since there's no PowerPC support at all, that means that Rosetta, the environment that makes those old PowerPC apps run on our Intel boxes, isn't installed during a normal Snow Leopard upgrade. If you still have any apps that absolutely require Rosetta, you can install it as an option, or it can be delivered via Software Update after the initial install. Even though you've probably already read this in previous TUAW coverage, Intel-only means that any of those G4 and G5 Macs that were able to run Leopard will not be able to run Snow Leopard. My G4 Mac mini will be stuck in Leopard for eternity, unless I choose to install Ubuntu Linux on it for the heck of it.
How can you tell if an application is PowerPC-only? It may take you a little while if you have as many applications installed as I do, but the best way is to go through older apps and do a "Get Info" (Command-I) for each one. In the screenshot below, this application shows the dreaded words "Application (Power PC)" as the kind of application. That means that if I need to run this particular application again after my installation, I'll definitely have to install Rosetta.
If you find relatively few applications that are PowerPC-only, it would be worthwhile to check with the software developer to see if they have a newer version available, or consider finding an Intel-based solution to replace the other app. That way, you can avoid having to install Rosetta -- not that it's a bad thing...
64 bits does not mean 8 dollars
In my continuing attempt to move all of the accumulated trivia from my brain into the minds of TUAW readers, that title refers to the old US habit of referring to a quarter-dollar coin as "two bits." Now that we're off the trivia, let's talk about the vaunted 64-bit capabilities of Snow Leopard.
At this time, Snow Leopard boots by default into 32-bit mode, so it won't take advantage of the huge addressing space that a 64-bit codebase can provide. That means that initially, most drivers and kernel extensions that were written for a 32-bit world will still work perfectly. Eventually, Apple wants manufacturers of hardware devices (printers, scanners, and the like) to migrate their drivers to work in the 64-bit environment, meaning that future versions of Snow Leopard or Mac OS X 10.7 may do away with 32-bit compatibility altogether.
Since Snow Leopard will boot into 32-bit mode, most drivers and kernel extensions will work flawlessly in the new operating system. You may want to check with printer and scanner vendors before doing your upgrade to Snow Leopard, just to see if a new driver is available or if they list Snow Leopard compatibility.
That being said, Snow Leopard does a much better job of installing printer drivers than before. Rather than filling your hard disk with gigabytes of drivers you'll never use, Snow Leopard will determine what printers you have used in the past or that are currently connected to your Mac, and will then install only the necessary drivers.
Notable application extensions that currently don't play well with Snow Leopard include 1Password 2.0 (you'll need to either upgrade to 1Password 3.0 when it is released, or follow these steps to get it to work with Safari) and the popular Mail Act-On (you'll have to wait for an update). While we haven't heard of many app enhancers that won't work with Snow Leopard, I'm sure that tomorrow will bring a new list of non-working extensions. Keep your browser pointed to TUAW during the first day of Snow Leopard for breaking news on what works and what breaks.
Sometimes it's the little things that you don't expect to create an incompatibility that will trip you up. System Preferences panes may do this to you! System Preferences is a 64-bit application, although it is written to know when you are opening a 32-bit pane and will relaunch in 32-bit mode. Although we haven't heard of any SysPrefs panes that are known to have issues, it's something to keep your eyes open for as you travel into Snow Leopard territory tomorrow or in the next couple of weeks.
Finally, most of your standalone applications should work flawlessly. The apps that are most likely to experience problems are those that address hardware directly. For example, I'd be wary of any apps that talk to scanners, DVD burners, or other hardware, or of any application that has been written use unsupported hooks into the OS. Once again, it's advisable to go out and check with the vendors of any hardware or software before you upgrade, just to see if they've already addressed Snow Leopard compatibility.
Best of luck with your Snow Leopard upgrade, and let us know if you run into any unforeseen problems.