At first glance it didn't seem like it had anything truly compelling (to me) to offer. A few of the features, mainly APIs, sound promising, but the whole "250 New Features" proclamation, like with previous updates is mostly a yawn-fest. But I think I can grow to like it, since I do like a lot of the cleanup they've done to progress bars, default buttons and the like. I especially like being able to resize windows from any side like MS has had for years.

I've installed Lion onto a spare HD and will be using it for testing, it's not ready for use on my boot drive, mainly because of third party app support.

While I dislike the default threaded (conversation) view in Mail, I do like the ability to instantly view "related" messages, even when you're not currently in threaded mode.

Here are some weak points:

Launcher app. Really, who cares? All apps at once? No thanks.

Mail's (default) conversation view is set on a per-folder basis, but there's no way to turn it off for all folders at once. You can select all folders and turn it off and it will appear to be turned off, but as soon as you click back to a single folder, it's ON again. You have to turn it off one folder at a time. This is going to take a LONG time (for me).

Auto-hiding scroll bars. How can you tell whether or not you should scroll up or down to reveal more content in a list? Sometimes it's easy, other times it's not. You certainly have no way of knowing where you are until you start to scroll. I've disabled the default in favor of always showing scroll bars (the old behavior).

Dashboard now has it's own solid background. Great, now when you need to reference something visible in another window on your desktop, you can't. Big reduction in usability.

While screen resolutions are getting higher (denser), someone at Apple decides to make GUI elements smaller. Smaller window controls on the top left not welcomed.

The default to flip pages which is also the default for BACK and FORWARD in the browser, uses two fingers on the trackpad. This is the same two fingers used for vertical and horizontal scrolling. Far too easy to inadvertently go back a page while trying to scroll the view. Had to disable this. I swear no one at Apple tests this stuff, just like magnification in certain apps with pinch - just not useful and a complete PITA.

The Finder... Wow, what a POS. Glad I don't use it and instead use PathFinder. If there was a way to step backwards while trying to go forwards, this is it. Reversed the side bar for no apparent reason and there's no way (that I can tell) to reverse it back. File grouping is annoying as hell with no rhyme nor reason for the order in which things are listed. Much much more...

Sidebar icons have lost all their color and contrast. Many windows are now starting to look amazingly bland, including the Finder's where color was a great cue to selecting icons.

Deleting an application that was already present on an existing system when updating the OS. Front Row is removed from your system when updating to Lion. I'm fine with Apple not including it in Lion, but I'm not fine with them removing an existing app. Nothing was mentioned in the install process about this, not even during the final bit about "incompatible applications."

The reason I'm not installing Lion onto my main boot drive is due to incompatibility with certain third party applications I rely on. Three right off the top of my head:

Glimmerblocker - doesn't work and the Lion installer actually mentions it at the end.

24U Appearance OSAX plugins for AppleScript - I absolutely need this day-to-day as part of a few Automator/AppleScript bundles I've created to help with certain business tasks.

1Password - I can't stand their new browser plugin. It may look flashy, but it suffers tremendously in usability and loses a number of features. The application itself loses a lot of browser integration features in this update with no set date for updates. IMO, stabilize the existing version first and then worry about new versions, making them optional installs. I rely on this to log in to a ton of sites and it's just cumbersome in Lion - including not working at all for some sites nor supporting HTTP Auth requests right now.