Why does iOS take so long to start up? My MacBook Pro running Lion boots in maybe 30 seconds. My original iPad, however, takes several minutes. Why is this? I understand the MBP has a much better processor and more RAM than iPad, but surely that can't cause that much difference—especially since OS X is vastly more complex than iOS.

Related: Why do iOS devices take such a long time to turn back on?
Well - at the most basic level iOS and Mac OS X are the same kernel and startup processes using launchd and standard unix-heritage programs.

iOS has full sandboxing and arguably tighter access control infrastructure to build up - so in some ways, iOS does more to start up than Mac OS X does.

Once you add on mac things, of course it is more extensible so many people have much bigger loads to lift on an OS X reboot than an iOS boot.

I would argue that all the above arguments are moot and to a first approximation, you are seeing the CPU capability of the Mac hardware shining as well as the read speed of the mac storage as opposed to the iOS storage and the effects of more RAM being a better advantage and more than offsetting the larger library and binary size on Mac OS X.