I know the TOS had to always live with a constrained budget ... but some of the attempts that got put on screen has me shaking my head even after all these years. For example, in "Return To Tomorrow," Thalassa sandwiches the good doctor between two horizontal energy "fires" that (when viewed in playback) look suspiciously like strips of paper flickering near a fan:

In "That Which Survives," Scotty has to work his engineering magic before the ship blows up. He's in the service crawl-way and the magnetic flow that make him itch with the feeling of ants dancing on him look like ... glowing blue tree branches?

In the same episode, a red shirt shoots a phaser at the Kalandan computer. This screenshot is taken after the phaser has been discharged for a while. It almost looks as if the batteries in the phaser are dying out. Starfleet apparently prefers the Kirkland brand.

Remember in "Friday's Child" when an enemy ship was confronting the Enterprise? We were treated to the sight of a Klingon ship for the first time in television history...

...which looks absolutely nothing like this:

One looks like a cool enemy vessel. The other ... a pair of scissors with a yellow glow around it. WTF.

Explosions are another area where they took shortcuts. In space, or practically anywhere else for that matter, you see shrapnel when something blows up. But in Federation space the laws of physics are apparently a touch different. You see, when energy weapons impact a ship, the ship bursts into fireworks and all the hull material magically disappears, usually even in the 24th century.

But in "Day Of The Dove," we were treated to a rare attempt to demonstrate ship panels flying apart:

It's not completely convincing though since the parts never flew beyond the debris cloud. It doesn't provide that gratifying experience when we saw the Borg cube blow-up at the end of TNG's "Best Of Both Worlds."