Its cosmetic changes are, in my opinion, dubious at best. Window controls are on the left hand side of the Window for no good reason. The color scheme is way too dark, it made me long for the old human theme. UI elements are glitchy in places. The "Me Menu" is useless to me. And why the heck did they ever remove the startup/shutdown options from the system menu? Why are they off on their own?
I like the idea of the Ubuntu Software Center but its execution leaves a lot to be desired. Last time I checked it was just another front end to apt. Many of the packages available are totally useless to the vast majority of people.
Ubuntu One and the Ubuntu Music store, well, we'll see how those go. Personally, I've never, ever used them and I have to wonder if they're the type of things that will actually draw people to Ubuntu
And, well, I hate to come off as a curmudgeon because I really do like Ubuntu, but to say that 10.04 blows OSX and Windows 7 out of the water is just wrong. It doesn't. For instance, why does Ubuntu still have the same old application menu? Both OSX and Windows 7 (and even KDE4) have moved away from this model. The categorized, hierarchical menus that have been around forever are really unhelpful. Why can't I search this menu like in Windows 7 and KDE4? For that matter, why can't I easily search everything like I can in Windows 7 and OSX (spotlight). If they really want to compete with Windows or OSX they need to give me better ways to find, sort and organize everything on my computer.