First off, I like the nice big Trash icon. It's so much easier to aim for when I want to to just drag n' drop to delete a bunch of files. I also like the screen space Unity provides especially when a full screen app opens and the launcher just automagically hides itself.
But after using Unity for about a month I think it's time I accept I should go back to Classic. Why? Well, here's my list.
Speed - Unity usually works fine, but sometimes it just decides to do... something. Whatever it's doing, it's thrashing my hard disk and my entire system just stops responding. Not good. Switching to Unity 2D greatly improved the situation but even then there's the occasional stutter. I've even blogged about it before.
Visual artefacts - Maybe the Unity developers just need to black list more hardware that can't handle Unity. Even Unity 2D has artefacts and it's not even supposed to require hardware acceleration (my understanding is that means it doesn't make use of OpenGL). Of course, I understand the blame's not entirely on the Unity developers. Linux GPU support seriously needs improvement.
Global Menu - It needs to be smarter, or it needs to go. Harsh, I know, but the way things work right now I have to use all apps full screen or it's confusing when I want to access the menus and it's not where it used to be (just below the title bar). Moving from a non-full screen window to the Global Menu just to access some simple menu item is... unproductive. Maybe the menu could go global when an app is running fullscreen but is back to its traditional place, otherwise?
Search - Oh boy. Why is it that the exact same term doesn't always find the same files? It's not like the files on my hard disk change all that much. Sometimes the same term doesn't even find anything. At all. But, backspace just one character and suddenly there's results. And no, there wasn't a typo in my search term. I double checked. If search is going to be central to how we do things in future, then it needs to be really good. Otherwise, there's going to be a lot of frustrations.
Those are my major complaints concerning Unity. I still believe there's a lot of potential here, but Oneiric needs to improve on Unity a lot if it intends to win over more supporters.