-Brief shots of the Rodan marionettes in the Japanese version of RODAN aren't too great. I can see why they were removed from the Kings Brother's version.
-The blue screen stuff in KKvsG is pretty rough. It's very noticeable.
-The Godzilla puppet in GHIDORAH. I never liked it, and like TVG said, it just takes me out of the action everytime I see it. The Rodan one wasn't nearly as bad. The shots of the Godzilla puppet should have been trimmed, much like they were in KKvG (several shots of that Godzilla puppet, which was even worse, were only seen in the original JP trailer)
-Most of the miniatures seen in the 90's films. When Godzilla's height increased to 100 meters in 91', it was all downhill from there..Also the sparks were very tacky. Always reminded me of Power Rangers.
-The CGI seen in G2K/GxM was just downright awful. Not much better in GxMG, but hardly cringe worthy. They got it right with TOKYO S.O.S, and even GFW has some impressive CG.
There are plenty of things to nitpick about some of the 70's films, but all of those were made with MUCH smaller budgets than any of the mentioned films, so they kind of had an excuse. Besides, listing all of the stuff from GIGAN and MEGALON alone would take too long.
Though I will mention the use of the Musuko Goji suit for the water scenes in GIGAN was horrendous looking. I guess the Daisenso Goji suit had just rotted too much by then, so they didn't have much of a choice.