Let me say this at the outset: software is the greatest strength of the Macintosh platform, and my primary motivation for buying the PowerMac. Between the high quality bundled applications and the wide array of available third party programs, the Mac has everything most desktop users will need. Since software is a rather personal matter, the best I can do here is to make some comments about the applications I use on a regular basis, and hope that the reader finds some overlap with his own usage patterns.
First, a meta-comment about how applications are handled on OS X. I'm on the fence about the DMG installation method. It is quite a bit more manual than APT on Ubuntu. I miss being able to update all my applications from a single interface. Further, since there is no way to install ancillary files along with a program, the documentation that accompanies the program in the DMG usually gets thrown away with the DMG. It is also irritating that there are several different ways of installing apps. Most use the DMG method, but Apple's own apps use a PKG installer. On top of that, Fink uses its own APT-like system. However, application installation on OS X is better than the cumbersome underlying mechanisms would suggest. The biggest advantage of OS X is that its developer base can, and does, target a single version of the OS. Furthermore, because of the expectations of users, developers take the time to package their applications very nicely. The difference between the two platforms is mainly noticeable with obscure software that doesn't normally get packaged by major distributions.
Okay, let's get the Finder out of the way. The OS X Finder seems to engender love-it or hate-it reactions, and well, I'm in the latter camp. I don't hate it, per-se, but I don't like it very much either. I have to admit that using GNOME for the last six months has really made me a spatial junkie, and I really do not like going back to the browser paradigm. Also, the OS X Finder really is as unpredictable as everyone says it is. There is a weird mix of spatial remnants in with the browser style. For example, if you click on "Macintosh HD" when you have an existing Finder window open, it will open a new Finder window unless the existing one has "Macintosh HD" open. However, it doesn't carry this exclusivity through the interface, since when you open another Finder window, you can browser to "Macintosh HD" in both. I could gripe about the Finder some more, but a far better reference for Finder's issues would be John Siracusa's articles about it at ArsTechnica.
On a happier note, let's consider Spotlight. Spotlight is an enormous boon for a user like me who hates organizing files manually. It's extremely handy, only a shortcut key away, very fast, and has an excellent success rate in finding relevant files. The fact that it searches file contents as well as file attributes puts it a step above the search implementation in BeOS's BFS. Live search folders are as handy as they were in BeOS. I use them to organize my e-mail messages, which allows me to find e-mails from a particular professor, for example, just by opening a folder. I keep trying to find more uses for Spotlight as I go along, but I'm already very satisfied with its performance. Apple did an excellent job of integrating search into multiple places in the UI, such as the Finder, the file dialog, and the menu bar, making the feature much more useful than it would have been with just a single point of access. I really like how Apple integrated meta-data annotations into the UI. The file information window simply has a plain text field named "Spotlight Comments". This simplicity makes a lot of sense. Given the users who will rely on Spotlight, the ones who don't organize their files into neat directories, files aren't likely to get detailed typed annotations. A simple free-form text field, however, has a hope of getting used.
Moving to the other much-ballyhooed OS X feature, Dashboard, I have to say that my reaction is: eh? I simply don't find it particularly useful, at least in its present incarnation. I don't need a computer to tell me that it's cold outside on a November morning in Atlanta. I don't need to look at the clock, because there is a nice digital watch right in the corner of the screen. I have never found a tedious on-screen calculator useful, preferring the real one on my real desk. I suppose Dashboard has potential, but as it is, it tells you either what you didn't need to know, or what you already did.
Next, let's consider Safari, OS X's standard web browser. Safari is, in general, a decent browser. Its rendering quality is fairly good, though not as good as Firefox's, and its integration into the overall environment is superb. Its interface is classically Apple, with a very minimal combo toolbar and address bar, and sparse yet eminently useful menus and context menus. It is configurable enough, and offers just enough features, like the integrated Google toolbar, to keep me from straying elsewhere. The RSS feeds feature, in particular, is very nice, and quite useful for those who like to keep up with Slashdot, OSNews, CNN, and other news sites. My biggest complaint with Safari is bugginess. Safari leaks like a sieve. I've been running it all day, and now with no windows open, it's using 230MB of RAM. Its also unstable. In the week, Safari has crashed on me at least three times. Further there are occasional visual glitches, such as bits of horizontal scroll bars "sticking" to the content area during vertical scrolling. Finally, there is an infuriating bug which will cause the browser to sometimes go back several pages after hitting the back button just once. Whenever I try to get back to my GMail inbox after reading a message, I'm thrown back to whatever website I was browsing before. Normally, I'd be quite pissed off about a browser that has crashed three times in a week, given that Firefox was very stable, but given the instability of recent versions of Firefox, I'm willing to cut Apple some slack. I am, however, like most OS X users, waiting for 10.4.4.
I was originally going to have separate sections for iTunes, Mail, and iChat, but then I realized: who cares? iTunes plays music, Mail shows you your mail, and iChat lets your friends send you funny links. They all do their job just fine, and behave just like every other jukebox/e-mail client/AIM client in existence.
The OS X terminal is quite a bit better in its Tiger incarnation than it was in early versions of the OS. It has proper clipboard integration and acts as a drag-and-drop target for files. It features command completion, a command history and decent amount of flexibility with regards to terminal emulation. Its interface leaves something to be desired, however. It doesn't anti-alias fonts by default, and figuring out how to enable the feature is non-intuitive. Terminal's preferences menu doesn't actually contain any preferences, those are in a separate "window settings" menu. Terminal's biggest weakness is that it is glacially slow. I thought GNOME's terminal was glacially slow, but OS X's redefines glacially slow. Let's just put it this way: one of my Lisp compilers has a warning in the documentation to not run the build from OS X's terminal, because Terminal's slow handling of the compilation messages actually slows down the build significantly!