When I purchased my first Chaosium RPG -- Call of Cthulhu -- back in 1981, included in that first edition boxed set was a little 16-page pamphlet called Basic Role-Playing (or BRP, as it's become known since). At first, I was rather baffled by its presence, since the box already included a much larger Call of Cthulhu rulebook, which I assumed included all of the rules needed to play. As it turned out, it didn't, or rather, the rules in the larger book built upon concepts introduced in Basic Role-Playing. Thus, to more easily understand Call of Cthulhu's rulebook, one more or less needed to understand BRP.
Young kid that I was, I thought this a rather odd way to present rules, but, in retrospect, the wisdom of Chaosium's approach was borne out. For one, reading and internalizing 16 pages (of which only about half consisted of actual rules) took no time at all, thereby creating a solid foundation on which to add further complexities. For another, it nicely emphasized what rules were the important ones, the ones on which everything else depended. The BRP booklet thus admirably served double duty as both an introduction to all the games derived from it and as a treatise on rules economy.
It's this last aspect of that 16-page booklet that's stuck with me all these years. In an age in which even "light" games are typically many times larger than BRP was in 1980, it's difficult not to admire the way that Greg Stafford and Lynn Willis distilled Steve Perrin's original rules design into its essential components. Re-reading it recently reminded me just how few rules are necessary to run a roleplaying game if those rules are well chosen and presented. There's little question in my mind that BRP is both, which probably explains why it's remained more or less the same in its more than 30 years of existence and has powered some of the most well regarded RPGs in the history of the hobby.
Quite an impressive feat for such a tiny ruleset!