Launchpad and app home screens
I wish I was excited by the demo of Launchpad, a feature that displays all installed applications as an overlay on the desktop similar to the iOS home screen, but I wasn't. The idea is to have all applications (presumably from the Mac App Store as well as other sources) available with one click -- and perhaps some swiping to see more app screens if they can't all fit onscreen at once. (You can get a similar effect by dragging your Mac's Applications folder into the Dock as I've done on each of my Macs.)
The concept isn't a bad one, but it's too easy to go from a good idea to a bad experience with it.
Any longtime Mac user might remember that Apple has tried this concept a few times over the years, first in managed workgroup and multi-user products such as At Ease and Mac Manager, and as an option in classic MacOS releases. In some cases, the MacOS Finder was completely replaced by a button-style view of installed or allowed applications. In others, a button view was introduced as an alternative. Perhaps the most commonly known and best-designed implementation was the Launcher. It should be telling that none of these solutions exists today.
Launchpad does seem an improvement over past attempts at streamlining and access restriction. (I imagine Apple will integrate it with parental controls and client management via Mac OS X Server and/or third-party Active Directory solutions.) But it could be just as confusing to some users as having to locate applications in the Applications folder and its subfolders.
I'm particularly put off by the idea of iOS-style folders in Launchpad. This seems to increase the potential for confusion and onscreen clutter, particularly if more traditional ways of launching applications remain: drilling down through folders, opening a specific document, keeping applications in the Dock and using Stacks in the Dock to browse through your Applications folder or other folders -- not to mention applications stored on removable media such as external hard drives, DVDs and network shares. Adding to all of those options, Launchpad with its various screens of apps and iOS-style folders seems to make application management more complex by the sheer number of choices rather than simplifying the process.
On the other hand, it also allows individual users to work in whatever environment they feel most comfortable. I think the key here is going to be how Apple ultimately delivers Launchpad as part of the OS. It should still be clear, at least to power users, where the applications are stored in the file system. Apple needs to give users complete preference in how they work with applications, regardless of whether they're installed from the Mac App Store, downloaded from another source, bundled with Lion or, in the case of users who upgrade to Lion, already exist on the computer from a prior version of OS X.
Mission Control
When I first saw the combination of features (Exposé, Dashboard, Spaces and full-screen apps) that Apple pulled together to create Mission Control, I was afraid that it could work out very badly. Having had a glimpse at the result, I think Apple did a pretty good, though not perfect, job.
I absolutely love the idea of swiping to move between screens (or Spaces); it's an effective solution for both full-screen apps and traditional windowed applications. I like that Exposé is incorporated, letting you see previews of individual windows and Spaces and switch among them quickly. I can see this combination working brilliantly.
I also like that it's a completely natural carry-over from iOS. Unlike with Launchpad, I think Apple nailed this marriage of features from Mac OS X and iOS, making a really useful new form of navigation.
The one thing that I don't get is making Dashboard a separate screen instead of having it appear on top of the current screen. To me, one of the big advantages of Dashboard is that I don't have to leave what I'm doing to check the weather, verify someone's phone number, control iTunes, make short notes to myself or do some quick adding/subtracting using the calculator. (And that's just a few of the built-in widgets; there are plenty of useful third-party ones as well.) I hope the fact that it was displayed in its own screen was just part of the demo -- or that Apple will change it back to its traditional mode before next summer.
The bottom line on Lion
Longtime Mac users (and even some new Mac users or Windows users thinking about making the switch) may be wary of some of the interface changes in Lion. A hesitation about them at this point is a perfectly valid. Apple clearly has some great ideas, but it will come down to how they're implemented to determine if they'll be reasons to celebrate or vilify Mac OS X Lion. And that's something we won't be able to judge for a while yet.