The Cage looks remarkably good (if slightly retro). I can only imagine how different the world would be right now if that was the path Star Trek had gone down.
Some aspects of TOS have aged poorly, but we are starting to get distant enough from the era now that its coming back around the other side as a sort of esoteric aesthetic. Until the space hippies episode.
TNG's first few seasons look awful and scream 80s.
The main problem with DS9 is the quality of some of the early episodes and Sisko's lack of facial hair. Odo's liquidity doesn't look great early on though. Really, it just needs a high def transfer. They don't need to redo the effects. Except to maybe add some facial hair to Brooks. If he wouldn't, you know, go completely nuts over them doing that.
I love to shout "Oh no! Janeway's being attacked by a special effect!" whenever I get a chance to see early, TV series budget, digital effects on VOY.
I am rewatching Sliders and, for at least the first two seasons, I am surprised how well it holds up, everything considered. Still, perms everywhere (highly noticeable early on) are a bit disconcerting. I had to check the production years because I had previously remembered those as long dead by '96.
Buffy and Angel are finally starting to age a bit.
Haven't watched any other older genre shows recently to comment further.