...And the roll of a die.
Traveller was always a system and a setting that appealed to me, yet always out of my grasp. It's very hard to play a game if you're the only one that wants to. But what really kept me was how the game played. I like my big bag of many polyhedrons, don't get me wrong, but the call of the days when only the six-sided die consisted my collection is strong. I've always been a great fan of rolling multiple dice in place of single, higher-value dice; it gives better results. Do take note however, that I'm not big on absurd rolls like 12d4 or 20d6, yikes!
From every mechanic, generation table, character statistic and action with randomized outcome, one would need nothing but the common die in classic Traveller. From shooting a walking bulls-eye to succeeding at one of the character's few trained skills, only two sixers were ever needed. This created a bell curve of success, which tied in fluidly with the human-centric feel of Traveller, in that we as squishy organisms usually only do so well, on average. The average roll of '7' guaranteed that characters still maintained a measure of success with basic skill levels. Because generally, we succeed at things we know (flat DC is usually '8'), while still allowing room for potential failure. A stark contrast to the very random nature of rolling a single d20.
Further on, we see the system continuing its variety of uses for the pairing of 2d6 with generation charts. Using one die in the tens, and another in the ones, to the similar effect of 2d10 for determining percentile successes. Each of these tables generate 36 possible results, which while small compared to 100 for 2d10, this again allows for a bell curve effect. The result places more common encounters nearer the center of the curve. So instead of truly random encounters in the sense, we can see an increased chance of more common mooks arising with more interesting and rare things turning up with a probability to match.
Maybe I'm still just too much of a wargamer at heart, but the simplicity of using fewer varieties of polyhedron while still maximizing their potential can be just as fun as throwing pyramids and buckyballs about the tabletop.