I actually sat and watched the movie from beginning to end this time (not so when I was a 13 year old newspaper boy who came late and left early to make it to football practice in 1969 (or so). Some things that stuck me this time (45+ years later):
(1) Thoughtful even if unpopular science like quiet motion of space ships (factual rather than the roar of ships in free fall around a planet in other sci-fi flicks). The techniques to simulate gravity or compensate for gravity free environments were consistent with reality, Even in the climatic scene where he enters into open space into a hatch with no little/no atmosphere is well done with great care.
(2) Visual beauty (alignment of moon, earth and sun shots) etc... musical arrangements make this movie unique. I actually enjoyed the dark screen with classical music at the end (something not so true when 12). I understand that this is not popular and forces one to experience auditory and visual beauty without "action.".
(3) Slow development allows for anticipation about what is coming next...and allows view to enjoy the visual artistry and music.
(4) In 1968 this film depicts people fixated on computer screens and distantly relating to others and appears to have captured how it has become (somewhat) today. The only thing missing is the "gaming addictive behavior" so prevalent now (along with urgent sky is falling now syndromes fed by the internet).
(5) The step by step extension into space is still reflective of how those analyzing such efforts (if humans reaching into space is a goal) still see it.
(6) there is still some presence of the family (mother and father singing happy birthday) though still a non-emotive response typical of technology focused society
(7) for those who understand artificial intelligence and large computer interfaced systems this movie seems more plausible now than it did when I was less aware of the implications.
(8) The movie leaves lot of room for the viewer to "fill in the blanks." A malfunctioning HAL is possible but so is the idea that contact with "intelligent life" was planned with HAL (another scenario) and the humans were seen as "expendable" and HAL may not have been malfunctioning at all! Computer first contact with intelligent life make much more sense..think of diseases, plagues, misunderstandings that come with species contact.
(9) Fast action, impossible physical events and continuously repeated adrenaline pumping scenes appear to be deliberately NOT in this film. Amazing that it is still around at all.