Jim was my first adult DM, running a game for my highschool age group because my good buddy Dave was his brother-in-law. And while Jim may have ran a sadistic killer campaign (he was the DM who decided to troll-rape a PC of mine), I learned a heckuva lot about DMing from him. One thing Jim taught me is that you can give the players wide latitude when building PCs and still run a tight ship. One of the ways he did this was by basically allowing 2 different sets of rules at the game table. On his side of the screen he stuck close to the core books and a few minor additions of his own, while the players cobbled together all sorts of mishmashed combinations of unofficial races and classes. And it worked, largely because of the cool air of detachment Jim maintained. He didn't care what sort of freaks we ran, there were two black puddings in the next room no matter what kind of character opened the door. This is what I sometimes call the "objective" approach to DMing. Under this method the Dungeon Master aligns his forces, builds his dungeons, and maps out his world with absolutely no consideration of what PCs or players will be interacting with it. To some that sounds like apathy but it really is an old school virtue. When we kicked in doors and threw down with monsters the players all knew that we weren't being treated like kids. We went up against the same kind of opposition that Jim's adult players faced. We lost a lot, but when we won the victory belonged solely to us.
Sometimes I still cringe when I look at experience point charts that are weighted based upon player levels or modules with great attention paid to Encounter Levels. To me it smacks a little of hand-holding the PCs. I miss the grand old days of vast and unwieldy outdoor encounter charts loaded with monsters too tough for your PCs to handle. Sure, stumbling into a two-hundred strong gnoll war party can cramp your style, but when you beat them the glory is boundless. That actually happened to us in Jim's campaign. We were around 5th level or so. As I recall I was playing Chester of the Pointy Hat, a magic-user. We fought so long I completely ran out of spells and spent a dozen rounds poking gnolls with my wee little dagger. The party archer ran out of arrows. Our side won the battle against overwhelming odds with only a handful of hit points left between us. The fact that it was a completely unfair random encounter only gave that fight more meaning to us. We took the worse Jim's world had to offer and came out of it alive.
Addendum to my Review of 5e's MM
57 minutes ago