either ebert missed it or I'm adding sophistry: I thought the third monolith, the one that comes down horizontally and neatly fills the screen as the warp sequence begins looking suspiciously like a big movie screen, represents the transmission of ideas as the next logical step from tool use. the fact that we are sitting in a darkened room having a shared experience of idea transmission is part of the message.

man the beast -> man the tool user -> man the thinker. HAL represents a failed attempt at this next step: trying to get our tools to do our thinking for us. dave enters the warp and observes first abstract images and then images of landscapes, but distorted from the look of a real landscape. we flash back to images of dave experiencing pain or horror. this process of having ideas transmitted to us, either as abstractions or distortions of real things is a traumatic one. It is, however, necessary on the path towards what we become next. Dave then dies as a human, and so too what we are now must die to pave way for the new.