DuracellEnergizer said:

I'm well aware that TOS used a lot of human aliens. My point, though, is that this can be excused because of budgetary/technological limitations of the time; I see no similar reasons for why most of the aliens on TNG looked the way they did.

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TNG had the same budgetary and technological restrictions that TOS faced. TNG had a bigger budget but it was by no means unlimited. Creating complicated non-humanoid aliens for one-off episodes weekly was and is highly impractical. CGI was still in its infancy at the time so there was no out there either.

And this is leaving out the almost impossible deadlines they faced for every episode. You can cheat sets quickly but creature effects are something else entirely.