Overall, I'm very happy with Snow Leopard, and loving all of the little refinements I keep stumbling upon. However, I want to mention a few pitfalls I ran into with my upgrade, on the off chance that anyone has a similar setup and might benefit from some advance warning, or any of the fixes I've found.
Safari & Input Managers
There were a few things we knew to expect (and maybe dread), mostly having to do with 32-bit vs 64-bit processing and compatibility. Input Managers were known casualties. Two of the major victims for me were my 1Password and Evernote plugins in Safari. In 64-bit mode, at least at first, they didn't show up at all. See the Switcher's Blog for updates on 1Password and Snow Leopard. The current 3.0 beta of 1Password is working for me in 64-bit, I just have some keyboard shortcut issues to work out. Hopefully the Evernote blog will start offering more updates soon.
In the meantime, there's a partial fix: if you select the application (Safari) in your Applications folder, go to the File menu and hold down the Option key while selecting Get Info, you'll get the Inspector panel (also accessible with a simple Command-Option-I). For applications which have 64-bit capabilities (such as Mail and Safari), you'll see a checkbox to force them to load in 32-bit mode. That gets Evernote working for me, so I'll run it in 32-bit until things get straightened out. I'll be keeping an eye on the Glims blog, as well.
Update: There's a press release & a blog post from Agile this morning that reiterate the company's compatibility stance for 1Password v2: it should work fine on Snow Leopard in all browsers except Safari, and will work with Safari launched in 32-bit mode. Version 3 will be fully supported with Safari in both 32 and 64-bit configurations.
I try not to load down Mail with a bunch of plugins, but there are a few I rely on daily. The two I'm missing most are WideMail and MailTags. The MailTags site indicates a compatibility release in September, and full compatibility (and new version) by the end of the year. That's going to be a long wait for me, but fortunately MailActOn has been updated and is working fine. In the meantime, WideMail users are losing not just their widescreen mail-reading functionality, but also their Subject column, which makes Mail pretty worthless. Dane Harnett, the developer, has posted the fix for the issue on his blog. It involves manually editing the preference file (plist) for Mail, but it's not terribly difficult. I'm hoping for more news on WideMail/Snow Leopard compatibility soon!
Custom Apache configuration
I won't go into a lot of detail on this one, as I'm still figuring out a few things. In short, my custom Apache/MySQL/PHP installation was broken after the upgrade. My custom build of PHP5 is hosed; I'm fairly certain it's another 32/64-bit issue. However, it looks like I'm going to be just fine running the stock version of PHP now. Snow Leopard includes PHP 5.3, and it's compiled with some of the important libraries that Leopard's PHP 5.2.x setup was lacking. It looks like I just need to step back and let Snow Leopard do its thing now. After removing the remnants of my entropy install, and reconfiguring MySQL (not included in Snow Leopard), I have a working dev setup for the time being. I'm noticing several missing symlinks in /usr/local, but they're easy fixes.
Scripting Additions, AppleScript's addons (osax) are running into major problems. Guess why? Yep, most of them don't run in 64-bit. Even if you've never installed any yourself, you probably have some from third-party applications (Adobe being a big culprit in my Console logs). This one isn't a big deal; it's more irritating than harmful as I weed through log messages and trash osax files. I don't think the errors cause a big CPU hit, but they'll trigger every time any application runs AppleScript. For now, I've moved everything except for Speed Download (which isn't causing errors) out of /Library/ScriptingAdditions. Check your system.log in Console.app to see if you've got some osax baggage to drop.
MindManager 7 for Mac
This one's killing me ... MindManager is completely unusable in Snow Leopard, and they're not promising a fix until September 15th. I know there are long lists of incompatible apps, but if I'd known for certain I'd be breaking this one, I may have made alternate plans. Wait, who am I kidding? I drove an hour to pick up Snow Leopard on release day because I was told my Family Pack shipment would be a few days late. I've learned to live with my eternal thirst for the cutting edge, so I'll work in MindMeister until I get MindManager back (nothing against NovaMind, iMindMap, XMind, FreeMind or any of the other great mind mapping apps out there, I'm just used to MindManager and MindMeister). I have to say, MindManager for Mac is overdue for an update anyway -- I've been thinking for a while that Mindjet has been doing a lot for their Windows customers, while Mac customers are sitting on a quickly aging (2 years since a major update?) version. The upcoming NovaMind 5 could quite possibly take over Mindjet's place in my heart, if they don't watch out.
System Services are the best and worst part of Snow Leopard for me, at least at the moment. I'm a total geek for custom Services, and I was running about 20 of my own creations up until the 28th ... when my entire collection was pretty well wiped out by the upgrade. I'll rave about the amazing new Services setup in a separate post, but I'll mourn the death of my old Services for right now. The power-up that the Services got is more than worth this temporary trauma, though, and I'm sure it won't be long before I'm looking back at this and laughing. I mean, you can build custom services right in Automator now, much to the detriment of my social life.
Honestly, the above has been the worst of it for me, and in the end it's been a great upgrade. My Applications folder is quickly coming up to speed, as well. A lot of apps broke on the 28th. In some cases, developers who thought they had achieved full Snow Leopard compatibility were bitten by late changes they let slip by. The fixes, though, are coming quickly, especially from the small-shop and indie developers. Thankfully, the vast majority of the software I depend on has been rapidly updated and last-minute fixes are out. Watching the MacUpdate and i use this feeds over the last few days has been a trip; programs I didn't even know were still being developed were up and ready with Snow Leopard-compatible builds.
Stay tuned, I'll be back soon with a cheery update on my favorite bits of Snow Leopard.