Well, you would slightly redo the races to add attribute penalties...
Dwarfs become Small Size with all attached restrictions.
All attributes are determined by rolling 3d6 in order of attribute. You basically need a 11+ in the right category to be anything but a Fighter. Actually good classes like Paladin or Ranger require exceptional stats of 14+ across 3 different attributes. If your main class attribute is 13+ you get a 10% experience bonus and if it is 17+ you get a 15% experience bonus.
The only classes everyone can be is Fighter (to a particular level, generally a single digit low level) and Thief (unlimited). The only other non-human can choose any other class is Elf that can be Wizard to a limited level. If any non-human wants to multiclass, they have to choose it at first level and divide all experience points by 3-- even after they hit the maximum level in their class.
People would be lucky to be proficient with even a single weapon, nevermind this whole category thing.
Create the whole Thac0 chart to count backwards and have AC equal to 10 minus your armor bonus.
Divide the saves into a bunch of really random and arbitrary categories which have different numbers for no particular reason.
No one gets skills except the Rogue getting a few, no one else is capable of actually learning to do anything and just makes raw attribute rolls.
People only regain 1 hit point per day without magical healing.
Everyone's character portrait ought to be a terrible lopsided sketch-- and everyone has a beard!
The haste spell ages you a year, Elves cannot be resurrected-- dead is dead. But, they are also immune to Ghoul touch.
Give all female characters a strength penalty. Gotta simulate how weak and useless women folk are.