All things considered I think TOS has stood up extremely well given it's 50 years old. But one small thing that always bothered me was beaming down to a planet that wasn't an Earth-like planet, mainly because they often lit the sky with a plain dayglo colour. Purple, red, green. I get the idea that some alien skies might be different, but it's like they would just stick a plastic colour sheet over the studio lights. Plus those planets would often be rock planets, which would make the colour more obvious. It is particularly notable in Spectre of the Gun where it's the same kind of set again, just with half built building fronts stuck on it.
