Ubuntu runs smoothly, with simple but nice effects, clean windows and (since 10.04) some very nice themes. Perfect? No, but definitely in the same league as OS X.
I'd say what is lacking most in Ubuntu is elegance, which I define as it having strong vision about how things should look and behave. On a Mac you can quite clearly see when an application does not fit, for example Google Earth and Firefox. I think this is in large part due to the elegance of the platform.
Ubuntu is also somewhat lacking in simplicity. I think in large due to their attempts to hide accidental complexity instead of working to fix it. This has gotten a lot better, but as an example, Ubuntu still spews files for applications all over the place instead of having them as an application bundle as on a Mac. Then you get desktop files to give you a single icon to click, but they hide the complexity instead of removing it.
Reading what I wrote I realize that there is quite a bit of overlap between simplicity and elegance. App bundles are both simpler and more elegant than spewing files for an application all over the place.