Lion has few pretty nice features, but it won't work like a charm. When I upgraded to Lion my Mac has been crashing more than ever before. Exposé was simple and useful feature, but Mission Control feels pretty confusing after month of usage. OK, exposé had few downsides like if you had opened 10-20 text documents it was almost impossible to find out which was which, but you could always press spacebar to open preview of window. That downside is also with Mission Control, because it has less display are and windows are smaller and harder to recognize.
Biggest issues
Mouse hover
Application won't recognize first click and they might do something unexpected if you press hotkey and it commands wrong application. This is also big issue with Firefox because sometimes hover navigation just won't recognize mouse on screen.
Mail
I haven't seen mail crashing never before. But after installing Lion it has done that several times. Sometimes it just crashes when I don't do anything with my laptop. It also can crash when I am typing mail. Once I typed pretty long message to my friend and mail crashed hopefully Gmail had backed up that text and I lost only one sentence.
iTunes
iTunes can also crash like mail, but that's not big issue, because it won't do anything useful except it plays awesome music.
Precision volume adjustment was great feature on Snow Leopard because it allowed to use in ear buds without becoming deaf.
It's not that bad...
Don't understand my wrong Lion is good operating system, but I don't think that it as good as Snow. Full screen apps are pretty nice feature on Lion I have almost always iTunes on fullscreen while I am browsing. Just swipe and you can change track on so on. Launchpad is also awesome way to launch applications.
I noticed after previous post that Logitech had updated it's drivers and there was now option to put Mission Control keys to mouse.
Hopefully Apple corrects these errors as soon as possible