Mythology
Saturn (Saturnus) is the Roman god of agriculture and harvest. His Greek equivalent is Cronus (Kronos), a Titan and not to be mistaken with Chronos, the personfication of time. Some might wonder why agriculture is related to Saturn, but it actually makes quite a bit of sense.
Agriculture and farming are continuous cycles of planting, growing, and harvesting. Rinse and repeat. In this way, it's a lot like ruin and birth, common themes associated with Sailor Saturn's role in the series. In some places, farmers burn away their fields to enrich the soil, killing any remaining plants but letting the soil replenish its nutrients. Then, the fields are ready to be used again for planting. Ruin and destruction are, indeed, necessary for birth and revival. Sailor Saturn does exactly this. Even her Silence Glaive, a sort of sickle or scythe, is a reference to Saturn and Cronus' roles as gods of the harvest.
Saturn is commonly associated with melancholy. An appropriate emotion for Sailor Saturn - for one thing, Hotaru is inredibly pessimistic and melancholic. Furthermore, as has been said a million times before, Sailor Saturn's job is a far cry from spreading bubbles and sunshine.
There has been some debate about the themes with Pluto and Saturn. Pluto is the Roman god of the underworld, equated with Hades in Greek mythology, commonly considered the god of death. As many fans think, Saturn is the one that brings destruction, and Pluto controls time, so this doesn't make very much sense, right? However, recall that Pluto's primary attack is called "Dead Scream." Just a little something to get you thinking. I don't consider myself qualified enough in the finer points of mythology to talk about this any more, so if you're curious as to more on the Saturn/Pluto issue, I'd recommend you take a look at the essays over at Dies Gaudii.