I'll tell you what started out wrong and they fixed it:
• The Keeper's voice in "The Cage" was originally just Malachi Throne. For "The Menagerie," they adjusted it electronically to be weird and alien sounding, and that was a huge improvement. Huge.
• The Ferengi started out as artificially mannered psychos with ape-like body language. That was insufferable and unsustainable. By the time DS9 got rolling, they were retconned into the best characters you could ask for. I loved them.
• TMP started off the film series with too much gray and pastels in the color palette. And the standard duty uniforms weren't very dashing. On Kirk and Scotty, the blue-gray version looked like nursing uniforms, or a toddler's footed pajamas. The Wrath of Khan had uniforms that projected more strength and power, bringing back the black trousers and a strong, vivid color on top. Not perfect, but a huge improvement.
Anything else?
• The Keeper's voice in "The Cage" was originally just Malachi Throne. For "The Menagerie," they adjusted it electronically to be weird and alien sounding, and that was a huge improvement. Huge.
• The Ferengi started out as artificially mannered psychos with ape-like body language. That was insufferable and unsustainable. By the time DS9 got rolling, they were retconned into the best characters you could ask for. I loved them.
• TMP started off the film series with too much gray and pastels in the color palette. And the standard duty uniforms weren't very dashing. On Kirk and Scotty, the blue-gray version looked like nursing uniforms, or a toddler's footed pajamas. The Wrath of Khan had uniforms that projected more strength and power, bringing back the black trousers and a strong, vivid color on top. Not perfect, but a huge improvement.
Anything else?