Unlike Snow Leopard - Mac OS 10.6 - the new 10.7 Lion release breaks a lot of old software. But that's not all.
5) Rosetta Rosetta is Apple's Power PC emulation software that works so well that you won't know how many PowerPC apps you have until you upgrade and they stop working.
I had replacements for my PowerPC apps - or can download open source replacements - except the world's best solitaire: Solitaire 'til Dawn X. Rick Holzgrafe, the developer, has an cautionary post on the lifecycle of the 20 year old app.
But you may have more important apps, such as old versions of Adobe Photoshop or Illustrator, that won't run on Lion. You can easily find out by selected the app, hitting CMD-i (or right-clicking and selecting "get info"):
If it says PowerPC it won't run under Lion.
4) Network Time Machine If your Time Machine backup goes to a network drive, Lion breaks it. The workaround is to attach a local USB, FireWire or Thunderbolt and use that instead.
If you use a local drive already, no problem. No word on when a fix will surface.
3) Broken apps Lack of Rosetta support isn't the only issue. Some apps, such as Nuance's Dragon Dictate speech recognition software don't work on Lion either.
You can check out Roaring Apps' App Compatibility Table, but don't take it as the last word: Dragon Dictate gets a "tested and has some problems" rating. It always crashes on startup for me.
Videobox from Tasty Apps gets an all-clear, but on their website they note that the Safari plug-in for Videobox is broken. Can still use the app, just not the easy plug-in.
Many pro apps are iffy. If you make a living on your Mac, wait until there are good updates for all your money-making apps.
2) Irritating changes and bugs There are some head-scratching "improvements" in Lion that - I hope - may get unimproved in a dot release.
For example, Apple has dumped the "Save As…" file option. Now Lionized apps have a "Duplicate" option: you duplicate the file - no built in keyboard command - and then Save it. Huh?
Apple Mail has - finally! - a horizontal letterbox layout. But despite what the preferences say, you can't enlarge the message list font. Weird.
Calendar and Address Book now have ugly leather-look interfaces. Can't believe Steve approved it.
1) There's no rush Lion will be around for a couple of years. 10.6.8 - the last version of Snow Leopard - works great.
Let other people take the arrows.
The Storage Bits take "Thou shalt not upgrade until 6 months after release" is the 11th Commandment in enterprise data centers. It isn't bad advice for consumers either.
Inevitably in any major software release their comes a time when release date and resource constraints force a reckoning. That's when cool features get dumped in a ditch and features that everyone assumed would be there go missing.
Lion is a worthy effort, but still young. Let it age on someone else's system and you'll be happier for it.
Comments welcome, of course. No, I haven't upgraded all my Macs to Lion, and I won't for some months.