I see a number of commentors over the year have felt the need to lambaste this perfectly innocent program. The central themes were about playing fair and making believe, and if it seems to be a simple premise, I'd like to hear where a sophisticated premise was used for a children's show that succeeded. The age group this show was created for was essentially preschool to kindergarten, and managed to coexist with the likes of Mr. Rogers and Sesame Street for decades, so it couldn't have been that gawdawful.
I seem to remember the show was a sweetly naive little half hour of kindergarten teacher types directing 'children' who were really toys how to behave together. This may regarded as insidious socialization, but it was created and ran through the very liberal 70s, so that claim is paranoid anti-government nonsense.
As for this show having no child actors, the toys made perfect surrogates, both because they were portrayed as childlike and because they reinforced the notion of abstraction necessary to allow children to see themselves in the same position. That level of abstraction was a necessary ingredient to instructing children to use their imagination.
As well, shows that primarily feature children often ran afoul of one of two problems with child stars: amateurs and professionals. The amateurs couldn't be relied upon to react properly to the puppets and toys, and the professionals come off so rehearsed and plastic as to be offensively unbelievable.
In the end, it's an argument over which philosophy for child education yields the best results. Personally, I don't think children's shows have been improved upon appreciably since the 70s, when at least diversity and imagination were openly encouraged, and the moral lessons were delivered a little more clearly without the obsequious and nauseating touchy feelie performances modern children's shows tend to use in lieu of actually explaining things to children. The assumption that children cannot or should not be told anything not relevant to playing in the schoolyard is utter nonsense.
With that in mind, I'd like to offer that The Polka Dot Door was actually a wonderful preschool children's show which hasn't been improved on by the likes of The Teletubbies.