Reboots Are For Upgrades

Linux enforces a strict separation between the kernel and other applications. Most services like mail, file and print serving, web serving and so on are applications, and can usually be changed dynamically. At worst, a specific application may need to be stopped and restarted, but not the whole system. This means that Linux only needs to be rebooted for kernel updates and hardware changes, and often not then. (This is a great boon for remote administration.)

For example, the Linux kernel is designed to use loadable modules whenever possible. If you hook up a CD-ROM drive to your SCSI bus, you can insert a CD-ROM device driver module into the kernel while the kernel is running and immediately begin using the CD drive. Of course, if you remove hardware you can also remove modules and reclaim the memory those modules were using.

While it is not recommended, there are reports of people swapping internal cards and drives without rebooting. Again, this is extremely risky, but it's possible under Linux. In Windows, installing a device driver always requires a reboot. (Once installed, some device drivers can be stopped and restarted.)

Linux provides the ability to dynamically increase swap space and then reduce it later without a reboot. This is in sharp contrast to Windows, where the swapfile can only increase in size, and never decreases until the next reboot.