Fixing the OS X Finder
The Finder in versions of Mac OS prior to OS X, sometimes referred to as the "spatial Finder," was a direct, simple interface for managing files: the Finder represented each folder on the hard drive with exactly one window, and remembered that window's size and position on the screen. This was part of the illusion of direct manipulation of files and folders— users would feel as if they were manipulating the files themselves, not representations or abstractions thereof. In fact, many users didn't even think of the Finder as an application at all; to them, it was the computer.
This merits a little elaboration, because many users don't see the advantages of spatial file browsing at all nowadays. The spatial Finder was so good because it allowed users to take advantage of the brain's excellence at spatial organization, and at direct manipulation. As an analogy, think of rearranging papers on a desk with your hands; nothing could be more natural, right?
On the other hand we have the "modern" file browser, which is the default mode for the OS X Finder. This kind of file browser offers many more features than the simple Finder, such as column views, the toolbar and location bar, and so on, but it is much more clearly an application through which users can manipulate files and folders. The extra layers of abstraction prevent the kind of transparency achieved by the old Finder.
To continue our analogy, think of complex file browsers as the office secretary. It's sometimes convenient to ask the secretary to organize or retrieve documents for you, since he probably has better organizational skills than you do; that's why he's the secretary, after all. But you wouldn't have the secretary stand next to your desk all day, doing everything for you; not only would that be a waste of the secretary's time, but it would actually be slower for a lot of tasks, since you'd have to tell the secretary what to do all the time instead of just doing it yourself. Of course, things are not nearly that bad with most file browsers, but the point is that direct manipulation of files can often be faster and easier than going through a heavyweight file browser.
The problem itself lies in the way the OS X Finder tries to accommodate these two methods as simply "modes" of the same Finder, because in reality they are different paradigms entirely. Not only is there no clear, consistent way to choose one mode or the other for all windows, but that would preclude using both methods seamlessly together anyway.
I suggest, as have others, splitting the Finder into two separate entities: the old-style Finder, and a new file browser with all the bells and whistles. Far from confusing users, if done correctly this would actually improve clarity and intuitiveness, mainly because of the transparency of the simple Finder as mentioned above. Users wouldn't be confused by the existence of two file browsers because to them there wouldn't be two: there would be the files themselves, accessible through the desktop; and then there would be the file browser, which would provide a powerful abstraction for viewing those same files. To aid this distinction, the new file browser would have a distinct visual look, as would things like Dock shortcuts to file browser windows (as opposed to shortcuts to files and folders themselves).
Better pundits than I have written very good articles on just this subject; in particular, see John Siracusa's article About the Finder... at ArsTechnica, and John Gruber's That Finder Thing over at Daring Fireball.