It seems that Windows needs to be restarted everytime there is the most minor hiccup. My experience with Unix (Irix 5.4 running on an SGI Octane) was that it almost never needed to be restarted. Being based on Unix I was hoping (expecting) that OSX would hardly ever need to be restarted. But I'm finding that not to be the case. Much less frequent than Windows but still some software installs, mainly those that the system itself calls for, require a restart. Can anyone explain this? (I'm not running SL, yet.)
Most of that behavior has to do with Wintendo storing all it's program settings in something called the "registry." This is a single huge database that stores the states for just about everything on a Wintendo machine, and is also a leading cause of BSOD's, difficulty in removing viruses/trojans, and the inevitable slowdown that Wintendo machines experience as you install programs and as you make changes. The registry is a critical part of the architecture though, and cannot be changed on-the-fly in most of cases, instead needing a reboot so that the new states can be read and loaded.
It was mentioned that other Unix-like operating systems don't need this, and this is largely true - there is no registry to keep track of everything; applications essentially have their own mini-registries in the form of text files that keep track of the settings for the particular program. These don't reside in the operating system directly, so there is no need. Linux seldom requires reboot after updating; I updated my PCLinuxOS install on my other laptop after almost two months. It found 256 applications that needed to be updated, but it did not require a restart; like vansmith pointed out, that usually needed only for kernel or major driver updates. The Wintendo install on the same machine had only 4, but did require a restart. I've found the Mac to be somewhere in the middle - not as frequent as Wintendo by a long shot, but more so than Linux or BSD. But like you said, stuff that the system calls for requires the Mac to restart, and I think that this has to do a lot with Apple themselves. Microsoft has always gotten praise not necessarily for the quality of their systems or applications, but rather for how tightly integrated they all are. There's always a very common look and feel to even very different applications. In their drive to be competitive with M$, I think Apple has begun integrating a lot of things, i.e. Quicktime, into the OS itself to maintain a highly integrated feel, which would necessitate new program states being loaded, but you can't do this while they're being used.
Sorry for going overboard on this post, I'm an engineer by training and I like to analyze stuff. :-D