OS X 10.7 means the end of Rosetta
With Lion, Apple makes a complete break from PowerPC Macs by abandoning Rosetta, the dynamic translator used to run applications written for PowerPC processors on Intel machines.
Snow Leopard downgraded it to an optional install, but it's been removed from Lion completely. This means PowerPC applications cannot run on Macs using the new operating system.
Apple famously described Rosetta as 'The most amazing software you'll never see'. It works so well you don't notice it's there, so it's worth checking if you're running applications you've long since forgotten were written for the PowerPC architecture.
From the Apple menu, select About This Mac. In the window that opens, press the More Info button at the bottom to open the System Profiler, showing even more about your Mac. In the left-hand column (labelled 'Contents'), open the Software list by clicking on the triangle to its left, if it's not already open.
Highlight 'Applications' and click on the Kind column header at the top of the window to sort your apps according to their type. Intel or Universal applications run fine in Lion, but PowerPC apps must be updated or replaced; they cannot be used with the new operating system.
OS X 10.7 Lion multi-touch support
Lion greatly expands the range of multi-touch gestures available to Mac users. The new OS adds a four-fingered pinch to open Lion's LaunchPad feature, an iOS-like application launcher. Unpinching with four fingers takes you back to the desktop, or if you're already there, hides all windows.
A three-fingered upward flick summons Mission Control, Lion's combined Exposé, Dashboard and Spaces feature, and flicking three fingers downwards gives a single-app Exposé. Another gesture carried over from iOS is Tap to Zoom.
With your pointer on a specific part of a web page, tapping with two fingers zooms in on it to give a clearer view. You can also enlarge and shrink web pages using pinch and unpinch.
These extra gestures will certainly be welcomed by Apple notebook users, and if you own a desktop Mac, it might be time to buy a Magic Trackpad. Apple's stand-alone device features the same multi-touch controls and glass finish as the trackpads built into the MacBook Pro range, but is almost 80% larger.
Like the Magic Mouse and wireless keyboard, it connects to your Mac over Bluetooth. If you're planning on buying a new iMac or Mac Pro from Apple's online store, you can opt for a Magic Trackpad instead of (or indeed as well as) a Magic Mouse, or you can buy one from your local Apple Store or reseller for £60.
OS X 10.7 Lion in the Mac App Store
So having established your Mac or Macs are capable of running OS X 10.7, how do you go about upgrading? Installing Lion is a straightforward affair. After purchasing from the Mac App Store, the new operating system downloads and installs automatically. It's as simple as that.
The download is around 4GB, which is about the same as a HD movie from the iTunes store, and after buying it once, you can install it on all your personally-authorised Macs. This is especially generous considering previous versions of OS X were also sold as 'family packs', with licences for up to five machines.
If you've already bought Lion and want to install it on another Mac, just open the App Store, sign in with your Apple ID (if you haven't already done so) and press the Purchased button in the top bar. Your previously-bought software – including Lion – is listed. Press the Install button and your OS is upgraded.