These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Space: 1999
Critical Research Failure: "The Metamorph" explains how an Eagle is desperately searching for the "rare metal" that is... titanium.
Crowning Moment of Heartwarming: John and Victor's warm conversation as the moon approaches the Black Sun and their conversation with the "cosmic intelligence" as they go through.
And the unworldly-sounding sitar score for "The Troubled Spirit".
Ensemble Darkhorse: Alan Carter was originally planned to be written out as part of the cast changes for Season Two. However, when the producers learned how popular he was with viewers, he was kept on and given an expanded role.
To a degree, Fraser in Series 2.
Fridge Horror: With every death of any of the irreplaceable 311 people on Moonbase Alpha, even Red Shirt characters, and to a lesser extent the loss of equipment, the bleaker the isolated crew's chances of survival become through the series even as they helplessly drift out of control through space.
Gets even worse when you think of episodes like "The Exiles", where a key plot element involves the fact that the Alpha life support system cannot support the 300 or so people on the base by that time. As a result, they are not only unable to take in the Galosian exile criminals, but they are also unable to permit any more births on the base.
Idiot Plot: the second season, especially - so much so that even Martin Landau complained (in particular, he hated "All That Glisters"note as did the rest of the cast so much that he threatened to quit).
At the time, it was the most expensive TV series ever made and the effects still stand up today, so this is probably an indication that critics didn't check their facts.
The spaceship fx are extremely high quality, and usually achieved using double-exposure rather than blue screen. This means the images are captured on the original negative and don't suffer from extra grain, although this does limit the angles that can be used. On the other hand the show does have some extremely poor matte paintings.
As has been said, the overall production value was very high, approaching feature film quality in many areas. The fly in the ointment is the writing; many cast members have made the point that the producers seemed more interested in the special effects than the scripts. That said, some of the actors rose above the material and there are some excellent character scenes, mainly those involving Barry Morse as Professor Victor Bergman.
There is a certain dissonance between good and bad special effects. Obviously, some viewers remember the good ones and other the bad ones. As an example, in the first-season episode "Dragon's Domain", we see a number of abandonded spaceships — created as very detailed and realistic-looking models — orbiting a supposedly Earth-like planet that looks more like a brightly-coloured plasticine ball (perhaps from a third-grader's art project) than any real planet.
The eponymous "Dragon" of the episode mentioned above is a very static Tentacle Monster that appears stuck in a doorway. In some shots it does look rather nightmarish, but the fight between it and the protagonist looks very much like a stage fight, with an actor trying to look like he's fighting obviously platic tentacles.
Just a couple of years later, the Star Wars movies came along and changed everything. Compared to Star Wars and later productions, Space: 1999 does indeed look a bit on the cheap side.
What The Hell Costuming Department: Despite the uniforms being designed by famous fashion designer Rudy Gernreich, they didn't look very cool or interesting even back then. If the intention was to avert the Space Clothes trope they succeeded only too well.
All of the scientific... ahem... inaccuracies would go away. A Wizard Did It!
Many of the plots are about inner journeys, mind control, possession - all classic Fantasy topics. As is fighting against the Monster of the Week.
And Professor Bergman would fit better as a wise old wizard than as a scientist - he's almost never shown doing any actual science, or rational reasoning, but rather seems to have some mystical knowledge of what's going on.