Really hard and often boring to watch. Most stories lack a clear goal or motivation. Things just happen. At the same time its full of overacting and empty dialogue.
Let me explain what I mean by comparing a Star Trek episode with a similar episode of Star Trek: The Next Generation: In episode S01E03 of the original Star Trek named "Where No Man Has Gone Before", the Enterprise happens to leave the galaxy and while flying through a nebula one of the crewmen is bestowed with supernatural powers and over the course of the episode develops extremely high intelligence and psionic powers. The crew immediatelly assume that he will become dangerous, talk about it, while he remains in the medbay and reads books at supernatural speed, can hear the thoughts of everyone on the ship, move objects around, etc. But there is no motivation to all this. He simply gets arrogant, forgets everything about being a human connection, empathy or his duties. The crew distrust him and think about killing him almost immediatelly. That itself is portayed as a shocking development. Then they decide to maroon him on the planet, where Kirk and him punch it out, another crewman weakens him and Kirk kills him and thats it. There is almost no substance, almost nothing to remember this episode.
Contrast this with a similar episode in Star Trek: The Next Generation, named "The Nth Degree". The basic premise is that the crewman Barclay that we already know to be very shy and awkward around other people gains supernatural intelligence. We know his normal traits from previous episodes and its also highlighted in the beginning of the episode. Therefore, its fun to see how he changes to an intelligent and confident man who charms Diana Troy. The crew is not immediatelly afraid of him and only gets hostile towards Barclay after he takes over the ship - it makes sense. His takeover happens because the Enterprise needs to solve a problem quickly and therefore he becomes the ship's computer in an emergency situation but then wants to stay connected, because his mind would not fit into his old brain anymore. There is a motivation for this - it makes sense. So he stays connected and takes over the Enterprise and makes the Enterprise travel tens of thousands lightyears where the crew and we find out that his transformation was initiated by those aliens who use this as a way to explore other races. They make the other races come to them, because the aliens don't like to travel. Not the best motivation, but its a motivation.
So after having watched ST:TNG it really stands out how most of the episodes of the original Star Trek are plain, boring, with overacted dialogues, with the obligatory fist fight and love interest of Kirk. I don't understand how anyone can like this show without a huge portion of nostalgia.
