AmigaOs boots so fast, because it's small, and relatively fast.
Linux distros such as kubuntu really do more during boot time. However, this is no excuse what so ever for taking so long to boot. Even winxp boots faster!
Ultimately a lot of pc software is just not very efficient, and this is caused by the 'if it doesn't run fast enough, use a faster computer' mentality, while on the amiga it's usually the opposite. Truth is, in todays world the power of pcs (and they ARE fast) is just wasted by running slow software (witch sometimes really sucks, as some of this slow software has great functionality).
A good example is adobe's pdf viewer. On my 550mhz p3 (winxp/linux) this program is so slow, it's almost unusable (both loading AND scrolling through the doc). The foxit pdf viewer (for winxp) on the other hand, loads in a second and scrolls very fast, allowing me to comfortably use pdf files. And I'm betting even foxit is not as fast as it could be.
The reason is thus: A lot of programmers today are screw-ups, or they just don't care...