I've been through phases where I've done frequent reboots. As noted above, hardware POSTs are usually the bulk of the cycle. There used to be other annoyances (Sendmail's 2-3 minute timeout on non-networked hosts was a real PITA).

Now it's other stuff. With chef and automated system management, repeat 'apt-get update' runs, which even with local caches and other tricks give about 120-150s per startup.