Yes, Expose' is really cool and functional. However, without mouse gestures to activate it or without a small set of buttons either in the notification area or the Dock, it makes it pretty far reaching for me. Call me lazy, but I prefer to use the mouse to do things rather than reaching the Fn keys on the keyboard (no, hot corners won't cut it for me, they activate Expose accidentally too easily and I want my mouse buttons assigned to their normal actions, not to Expose'). Here is a mockup of what I would like.
Virtual Desktops (workspaces)
As I said above, Expose is really cool, but for some advanced users it does not completely take away the need for virtual desktops. This is because it is very convenient to have a full screen Windows XP desktop on one virtual workspace, and then have another for X11, and another for email and another for casual browsing or chatting, and another for real work. It just helps out organizing better your work with the computer. There are a few third party utilities that enable this function, but they all have their problems, mostly with X11's window manager. If Apple was to incorporate such a feature on OSX, I am sure they would have all the developer support they needed to make it work properly. I see Expose as a complimentary to workspaces, not as an replacement.
Reach same applications' windows via Command+TAB and Cursor Keys
On Panther, by doing Command+TAB (equivalent to ALT+TAB on most PC Oses), you get a nice-looking transparent window with the icons of the applications currently open. It allows you to select and focus on another open application. However, what it does not do is let you select individual windows that belong to the currently selected app in that application-switcher window. On BeOS, you could use the cursor keys (Up and Down) and navigate to the open windows of the same app. Sure, Expose can do that even prettier, but if Expose was the answer to everything, this application switcher utility wouldn't have made its way to Panther in the first place.
Since I tried Path Finder, I fell in love with its main feature: Path Navigation. You basically get some buttons automatically added to its window every time you navigate inside a new folder. By having the whole way in the form of small buttons, you can navigate to that already walked path, back and forward, extremely quickly. Much faster than the normal "Back" button.
Fast User Switching Icon Item
I have being talking about this in the past as well: I would have preferred the option to have a small 16x16 icon on the menu bar that, by clicking it, would drop down the list of users, instead of displaying my full name on the menu bar. Even on 1280x1024, it does not leave enough screen space for some demanding apps that span across 12-14 menu items. It's bad enough for my name, but I imagine that someone with a full name like Alejandra Francesca Rodriguez López de Medina won't be happy at all to see that on her 1024x768 iBook or iMac...
This is mostly a gimmick, but Apple is known for gimmicks, so I am sure I am talking to the right people. The idea is to have "live" backgrounds instead of static pictures. Of course, instead of destracting objects and animations (people with no usability in mind can easily create those, there could just be some "lite" motion Flash or 3D movements, e.g. a waterfall, or a picture of a mountain under the snow which gradually changes depending on the time of day (e.g. it would look different in the morning, different in the night etc). Other more useful applications of this idea would be to have a picture of a rotating Earth with light on the parts of the planet that it is currently day (like XEarth). I am sure Apple could find some great ideas to further enhance the "wow" factor of Mac OS X; they seem to like stuff like that at the Infinite Loop... A more practical way to do this would be to allow screensavers to run in place of background images (development-wise they are the same thing). This should be easily done with a Finder hack, but unfortunately the vast majority of screensavers available are not suitable for such task as they are all destructing and flashy.
Some Theme Support
Please let me make myself clear about this: I don't like themes too much. I don't like the user ability to download buggy or butt-ugly themes and install them and then brag how great (==different) his desktop looks like (usually it doesn't). However, some themability is not a bad idea. Especially if the color and widget themes in question are made all by Apple, are well-designed, have being tested for usability and accessibility, and are consistent across the board with Carbon, Cocoa, X11 and Java apps. Then, in that scenario, I say 'yes!,' bring it on. Otherwise, please leave it as is. 'Less is more' anyway.