While the OS is reasonably stable, the UI changes in Lion make it a lot harder for someone who was used to having a powerful UI that was reasonably simple.
- Mission Control. Some people like or don't have a problem with Mission Control. Personally, Mission Control sucks and is a huge downgrade from Expose/Space. I can no longer see everything at once because windows cover one another. I can only Expose one Space at a time. Spaces on segregated alone when multiple displays are used. The Spaces icons are now extremely tiny, making it even harder to see what is on each Space and forcing me to swipe through 4-5 spaces to get a clear vision of where things are located.
Maybe this is fine for you. But it sucks ass for anyone that was used to having 40+ windows open at at time spread across 4 - 6 Spaces.
- iCal is remarkably stupid this time with even less functionality than in Snow Leopard combined with a santorum-nasty skin based upon a stupid desk calendar.
- Address Book is now crippled in order to look like my mother's old address book and can now only show 2 columns of information at a time. Thanks for reducing functionality just so Apple can make my computer look something old. Maybe Apple can have Jon Ives put in some vacuum tubes into the hardware to pay homage to the inefficient past.
- The sidebar icons in Finder are now grey, making it harder to find the things I'm looking for (although there are simple hacks to bring them back)
- The "Duplicate" instead of "Save As" is annoying because now I have to selected the old window and close it before I can continue my work
- Versions adds a lot more steps to doing simple, temporary changes to documents (such as checking a formula change in Excel) and heaven help you if you made a change that you didn't want saved and weren't really thinking about.
While Lion may have added a lot of new features, the UI decisions used to implement those features seems poorly thought out and not on par with the level of detail that I've come to expect from Apple products.