i wouldn't say old school d&d is "rules heavy" unless you're talking about ad&d  old school d&d is procedure heavy  > overland travel procedure  > dungeon crawling procedure  > random encounter tables  > encounter procedures (before combat) it gave the dm a solid framework.  nuskool literally teaches dms to fucking pull shit of their ass. see the examples of tracking time in 5e.  exploration, one of the three "pillars of play" as hasbro calls them, is meaningless in 5e without hard and fast procedures for movement and time management.
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