Some of you may have noticed that I almost never refer to a non-D&D game as "old school," or rather that I only use the term Old School as an adjective for the type of D&D I play. It's rare nowadays that I play a non-D&D rpg and when I do they are almost always one that is in print. I look at most rpgs as just rpgs, and only need the OS signifier to note that when I say D&D I don't mean 3.x or 4e.
However, I've recently had the chance to look over Traveller and I'm beginning to wonder if there isn't something different about the way it's set up that makes it similar to OSD&D but different from World of Darkness or even Call of Cthulhu. OSD&D and Traveller and a number of other "old school" games contain a large number of tables for determining the nature of the milieu the PCs will be adventuring in. More modern rpgs tend to assume that the setting and encounters are either lovingly crafted by the GM or spoon fed to you by the publisher.
Personally I absolutely adore random tables in general but go particularly gaga over encounter tables. In my creative, but at admittedly stunted, mind these tables act as the physics of the universe I'm presenting. Of course I customize them, bend them, and even flat out ignore them but I'd definitely rather have them than not have them. It's even better when, like the tables in the Monster Manual II, the author illustrates how to customize the table to fit the particular milieu the GM is running. That's just goddamn delicious.
Traveller of course has tables for generating is subsectors, stats for individual planets, and random encounters. In that way I find it almost superior to D&D. While it's unlikely that it will ever replace D&D in this sword & sorcery lover's heart, I do wish D&D possessed tables for making hex maps and such. I would add one to my still exceedingly hypothetical Uz supplement, but I'm too dependent on the Welsh Piper's wonderful system.
This has become my biggest "fear" for the DCC RPG.* The beta version of the rules states several times that it assumes the "judge" is capable of stocking dungeons and determining what monsters live where. Personally, I miss the tables. Sure they results are often goofy -- such as the owl from the last session of Nightwick Abbey -- but they provide a sense of phantasmagoric verisimilitude that is utterly lacking without them. I dearly hope they add some, because without them I have little reason not to just continue playing AD&D.
I want to make it clear that this is not the only qualification something should have to be considered "old school," and I definitely don't require it for me to enjoy the game. For example CoC is one of my favorite games of all time. However, I do more or less require it for any game I run as a campaign. It provides the laws of the campaign's reality and I greatly enjoy that. If I end up creating a product or free download you can bet it'll have a number of such tables in it.
*I put fear in sarcasm quotes because at worst it means that I just won't purchase it.