I recently got a new Mac Mini. I love having so much computer in such a small, quiet package. But I do have a few gripes about changes between Leopard and Lion, mostly having to do with differences between Spaces and Mission Control.
First, when I switch between apps (using cmd-tab or by clicking an icon in the dock), if the app I'm switching to doesn't have a window open in the Space or desktop I'm currently on, Leopard would take me to the Space where I was last using that app. Lion takes me to an arbitrary desktop where the app has a window open. Since I tend to keep Finder windows (in particular) open in multiple desktops, this makes cmd-tab and clicking the Finder icon useless for switching back to the Finder. Instead, I have to either use cmd-arrow go to Mission Control and click the desired desktop.
Second, with Spaces, I was able to drag windows from any Space to any other Space in the grid view. With Mission Control, I'm only able to drag from the desktop I was last on to other desktops. So to move a window, I have to go to the desktop where it is, go to Mission Control, and drag it to the place I want it.
Finally, although I have the same amount of RAM in this computer as the one I ran Leopard on, and I usually keep the same apps open, I'm frequently kept waiting while switching between apps -- presumably while waiting for them to swap back into RAM from the hard drive.
Looking at Activity Monitor, it seems there's a good sized chunk of memory being using for caching. Hopefully it's being put to good use keeping apps running fast, because otherwise, more of it ought to be used to keep apps in RAM for faster switching.
In any case, Lion definitely needs more RAM than Leopard did. I'll have to upgrade soon.
Other than that, I'm pretty happy with the new Mac!