The first season was surely one of the most unique sci fi series to ever hit the airwaves. The scripts were clearly morality plays, more interested in saying something than in being accurate science. In fact, the poor science and logical flaws apparent in the scripts is often the shows biggest drawbacks. Fans were left scratching their heads and wondering how X, Y, or Z happened. If you expect hard core science in your sci fi, then avoid this show at all cost, but if you don't really care, this is a great show.
Essentially Space 1999 (the first season) was a philosophical show about destiny. God (usually treated in a new age manner) was a central focus of the show. The inhabitants of the moon were not truly in control of their fate, not matter how hard they tried. Each week they would encounter events which made for morality plays. One week, they encountered an immortal man whose immortality made him a madman. The next week they would find a group of travelers who had resorted to cannibalism to survive, loosing their very humanity. Still another week they would find that paradise cost far more than they are willing to pay.
The best episode from 1st season were "Death's Other Dominion," "End of Eternity," "Mission of the Darians" featuring Joan Collins, and "Dragon's Domain."
The first season was done by Gerry Anderson and was originally a follow-up to his UFO series, but Americans were not familar with the show so they dropped the crossover, except for the moonbase itself. Ultimately, the logic flaws in the scripts led to so much criticism that the studios demanded major changes for second season. Those changes killed the series.
The second season was dramatically altered and the original producer left the show. The result was a blatant Star Trek rip off with atheistic philosophy borrowed from the worst Star Trek episodes permeating the show. One episode even features Commander Koenig uttering Captain Kirk's famous lines where he shouts down "god" declaring that the serpent was right to give us the apple! A new character was also added who could transform into anything including rats! Ick.