This is a matter of record: At the film's 1968 premiere, 241 people walked out — including Rock Hudson, who was heard to say, "Will someone tell me what the Hell this is about?," or, more colorfully, "What is this bulls***?" Although that may be apocryphal. 

Indeed, the general atmospheric attitude immediately after the premiere was that Kubrick had made an impressive technical achievement without making an actual, y'know, movie. This was not traditional mainstream entertainment, but a heady and ambitious work of art and philosophy. Which, as a cynic might take as a given, was not embraced by mainstream audiences. Kubrick ended up shortening the film by a good 20 minutes after the premiere. According to those in the know, all that was lost was a second Pod sequence similar to the first. As it stands, the 140-minute version doesn't feel incomplete. 

Most critics at the time noted "2001's" notorious opacity, whether to praise or lambast it. The New York Times review of it placed it somewhere between majestic and boring. Roger Ebert noted that there were many walkouts at the premiere (which he attended), but defended the experience, knowing that those with patience had witnessed something profound and significant. Critic Joe Morgenstern of Newsweek was particularly harsh, however, saying the film begins as "a whimsical space operetta, then frantically inflates itself again for a surreal climax in which the imagery is just obscure enough to be annoying, just precise enough to be banal." I admire Morgenstern, but ouch. 

Anecdotally, this has been the reaction from many peers of mine throughout my own life as well. Some friends and colleagues have said they love the music, the imagery, and the groundbreaking special effects in "2001: A Space Odyssey," but begin to flounder when asked to explain the film's themes or even basic elements of its story. As one friend pointed out, it's the only sci-fi epic with an extended period in the pre-human world (although since that statement was made, we also witnessed Terrence Malick's "The Tree of Life"). Another peer, to this day, calls the film the biggest waste of time he's ever spent in a theater, and this was a friend I saw "Spawn" with. 

The 1968 review in The Guardian exemplifies the mixed attitudes of the time, calling it beautiful to look at, but missing a certain spark of originality, and most certainly impossible to penetrate. The critic felt that Kubrick and Clarke hadn't thought things through.