Don't stop now. Easier to rebut all at once.
OK. More fodder to wade through.
Classes restricted to alignment. Rangers can only be good. Why? Because I guess it takes a strong moral compass to survive alone in the woods... you know, among all those moral animals. Henchmen for some classes have to be a specific alignment too. motive, story, party resources - that stuff is inconsequential. Extra manpower is needed to defeat this evil but Neutral Good isn't Lawful enough to come along.
Alignment languages. How...wha-..I can't wrap my head around what the hell Lawful Neutral is supposed to sound like ("If I don't survive, tell my wife 'hello'"? Something like that?). Just dumb... an archaic vestige of the days when alignment was important for no reason.
Finally found the Bard class - at the very back of the book, well away from the other classes. OK, maybe this is more of an indictment of Gary's technical writing skills and shitty book layout.
Oh boy, rules about how paladins can't own anything. That was fun, eh? Ah wait, there's a bunch of caveats too! They can own (but not exceed ownership of) 1 suit armor, 1 shield, 4 weapons, 4 other magic items. What happens if a paladin with 4 weapons picks up a club he finds on the ground? Well, Jesus compels him to drop it, obviously.
Hmm, rules for a paladin's "holy sword". Oh, it's just a bunch of pointlessly tacked-on abilities whenever a paladin has a sword designated as "holy" by whatever metric one measures such things. Circle of power 1" (sic - 1 inch?) in diameter, and can dispel magic. Maybe just make that shit part of the sword instead of a class bonus ability for the sake of simplicity? No? Ok... you're the expert after all Gary.
Chances to track as the Ranger - people complain 3e had too much math, then they employ systems that add +2% to chances of success. Ugh, these minutiae are endless and pointless.
Multiattacks at the mid-level are 3 attacks every 2 rounds. Extra layer of things to keep track of. As a DM, keeping my players adherent to this rule will be annoying as shit. "John, did you use two attacks or one attack two rounds ago?"; "I don't remember".
Multi-classing is for non-humans. Dual-classing is for only Humans. Why? Because reasons.
A table for Minimum Fees for Assassination. It's nice that something like setting your own contract prices and tense negotiation has been taken right off the table, otherwise we might be having fun playing an Assassin character. No Assassin could ever work for a discount, a favor, or any other reason - that sort of dynamic character-driven story doesn't belong in this game.
While we're looking at adding mechanics to replace roleplaying, let's also look at Loyalty Base stats on the Charisma table for NPC followers. Roleplaying NPC loyalty? Building bonds through shared experience and dialogue? Nah, friendship/respect/pay means nothing if you can't track loyalty with some hard, concrete, situation-independent number system.
Surprise in 5e: Perception Skill check to determine if surprised. Roll initiative. Surprised characters/creatures cannot act on the first round of combat.
Surprise in AD&D: Calculate Surprise Dice Difference to determine number of Lost Segments. Determine which of either party is surprised through contested 6-sided die roll using a baseline value of 2-in-6 and applying esoteric modifiers. Check DEX table for Reaction/Attacking adjustment bonus. Run through segments making partial actions, probably forgetting to apply the DEX bonuses that exist for this very specific situation. Then roll initiative.
Shields give you an armor bonus... up to a certain amount of attacks per round, which is annoying. Normal shields apply an AC bonus only twice per melee round, so if you've got a shield, you'd better remember that this rule exists and apply it's bonuses sequentially in combat, otherwise you're a cheater. There's nothing like nitpicking the use of a single AC point in a busy combat situation to really slow the pace!
Descending Armor Class. 'Nuff said.
Fire Rate and Armor Class Adjustments scores to track for ranged and thrown weapons. You know what's fun about having to refer to a book table every time you attack a different target or somebody moves around? Nothing, that's what.
Ugh, and I've barely left the character creation part of the book.