TAS to me is like illustrations from 1950s and 1960s books about space suddenly coming alive and turning into a motion picture. They have this unique feel to them, one of the reasons being probably the un-restricted colour range - nowadays cartoons, adverts etc. look as if the world only had 256 colours or maybe even something like 16. Red is only "red scraming at ya", there is no crimson, orangered, salmon etc. Everything has become strangely restrictive and monotonous colour-wise while those old paintings and illustrations show what the world is really like, they have this unique atmosphere.
Another reason why I like TAS are all those things that would not be possible or rather difficult with organic actors. For example Lieutenant M'Ress! Or all those strange animals they meet on other planets. People's imagination regarding alien fauna and flora still seems to be awfully limited so it is nice to see some creative thought here.