Interesting question. Even when I first saw the movie ( when it first came out, I was in my early twenties) what struck me was not so much the technology, as the logistics. Heywood Floyd flies to the space station, and later to the moon, on empty spacecraft; he appears to be the only passenger. Even given the high security of his mission, this struck me as a colossal extravagance: both craft were equipped to carry many passengers. Except for staff, the space station seemed to be mostly deserted (excepty for those snoopy Soviet scientists!). The lunar landing stage was not so much technically unbelievable as it was unnecessarily elaborate. The ship lands on an elevator platform and is then lowered into a vast underground cavern. Surely a concrete landing pad and a ground car ride to the terminal would have been more than sufficient.

The transporter ride to Clavius was, to me, the only technically puzzling aspect of the film. It would seem to be highly energy-inefficient to support the vehicle off the lunar surface and then travel parallel to it. It's convenient for aircraft to move that way, but on the airless moon, a propulsion system similar to a spacecraft is what you need, something to boost you into a parabolic trajectory and then brake you at the end: like a rocket. It would have made much more sense if they had traveled on a land vehicle, or even something like a train. Oddly enough, the cramped confines of the transporter came across as very realistic, like riding in helicopter.

In the same vein, the spacecraft used to travel to Jupiter seemed to be a lot bigger than it needed to be. The volume needed to support a crew of 5 (including the guys in suspended animation) could easily have fit into the pod bay alone. If we take modern nuclear submarines as a guide, men can comfortably live in confined spaces for months quite easily. Comparing these to older, diesel boats, they can even tolerate highly confined spaces if properly motivated and trained. That's less volume that needs to be life-supported, and less hull and structural bracing needed to contain it, yielding substantial savings of mass to be accellerated. In fact, all the vehicles in the film seemed to be a lot bigger than they needed to be, and highly underutilized. I understand there are dramatic and art-direction reasons for this, but I'm surprised that the effect of this on technically sophisticated viewers was not anticipated by a filmmaker who had obviously done his technical homework.