Well, the first season did kind of settle into the "little people captured and everyone else has to climb and climb and climb for 45 minutes to rescue them" formula that made the latter half of the first season pretty dull. The second season sort of started the same way but then got "Irwin Allen wacky" with time travelers, radiation hallucinations, clones, and secret cities. This actually made the series a lot more fun for me.
It is that wackiness I'm talking about mostly. Without Smith, Lost in Space probably would have been a weird show with eccentric aliens either way, but played straight by the cast, leaving the absurdity to the situation instead of the reactions of the characters. Admiral Nelson vs a spy and Admiral Nelson vs a flame monster: the only difference was the adversary. Basehart played them equally straight, but the latter episode was still weird and considered pretty danged silly.