In addition to short rulebooks, the other thing that strikes me about those early games were the relative lack of supplements aside from adventures. Most RPGs back then never got a rules supplement of any kind. There was never an expansion of Gamma World, for example, and I can't say I ever felt the need for one. Even AD&D was essentially complete rules-wise after the publication of the Dungeon Masters Guide in August 1979 and there were no significant additions/alterations to its rules until the release of Unearthed Arcana in 1985.
Part of the reason for this, I think, is that much of what would go into a supplement nowadays was instead presented either as a small add-on at the back of an adventure or in magazine articles. Back in the day, there were lots of new options offered up in the pages of Dragon, often under the byline of Gary Gygax, and we picked and chose which ones we wanted to use in our home games. My dislike of cavaliers, to cite one example, was solidified years before the class officially appeared in a D&D product, since I'd tested out the class in my campaigns after having read the original presentation of it in Dragon. Those articles served the purpose of keeping the game "fresh" through the introduction of new ideas and concepts, but, because none of them were formally introduced into the canon of the game, it was very easy to prevent their destabilizing its core.
Again, I'm not quite sure when things started to change. I know by the mid-80s it was much more commonplace to see regular rules supplements for many games. Indeed, it seemed to be that publishers felt that nothing short of a rules addition warranted publication and so supplements became the bread and butter of game lines rather than adventures as they once were. There are still a handful of games out there nowadays -- Call of Cthulhu springs immediately to mind -- for which rules supplements are largely anathema. I can't help but think the shift away from adventures and toward regular rules expansions is another bellwether of the end of the old school.