Some people rag on the animation style, but it's no better or worse than anything else on Saturday morning at the time. Star Trek was still about ideas rather than big action scenes then. Even with limited animation, they were able to have aliens and vistas impossible to realize on the live action series.

Interestingly enough, Lost in Space was tried in animation around the same time. The end result barely resembled the series, (even with Jonathan Harris voicing Dr. Smith) and only a pilot episode was ever made. It aired just once as a tv special.

A couple fan productions have made TAS episodes using modern tech to replicate the look and feel of the original animation.