What is old school style play?
A concise definition is quite difficult and subjective, but I like how Ben Milton characterizes it here:
“The more of the following a campaign has, the more Old School it is: high lethality, an open world, a lack of pre-written plot, an emphasis on creative problem solving, an exploration-centered reward system (usually XP for treasure), a disregard for "encounter balance", and the use of random tables to generate world elements that surprise both players and referees. Also, a strong do-it-yourself attitude and a willingness to share your work and use the creativity of others in your game.”
These elements were present in the early years of the hobby. At first, they may seem crude, cumbersome, or outdated. But many players find the kind of gameplay that emerges from these elements very compelling. They can invoke creativity, engagement, wonder, and a sense of accomplishment.
The appreciation of old school style play shouldn’t be misconstrued as chasing a sense of nostalgia. That can be true for some, but an increasing number of players who enjoy this style weren’t alive when these rules were popular. And beyond simple retro-clone and rehash rulesets, there are many elegant modernizations, reinterpretations, and entirely new and innovative rulesets that foster this style of play, and even more that permit it.