Television pilots have the thankless job of setting up a potential series in less than half the time than a movie usually gets to define it's premise and characters. Had it been a 90 minute feature film, it would have been head and shoulders above most other SF films made before 2001. (One can also see the influence Forbidden Planet must have had.) This was a very expensive pilot for it's day, and what happened after NBC rejected it was unprecedented.

It only suffers in comparison to the 78 Shatner led episodes that followed.

When I was a kid, and only knew of The Menagerie two parter, I really thought they had gone to the trouble of changing the sets and costumes to create the "past" scenes.