Having roll under stats was encouraging everyone to just rely on those skill and stat rolls. A classic character abilities overshadowing player skill situation, where rather than interact with the game world by, well, interacting with it, things got boiled down to rolls. Naturally some players already had 'player skill' and didn't succumb to this, but the mechanics need to actually inform gameplay or they need to be pitched. Similarly, using 'roll under stat' made stats actually pretty important- even MORE important than even 3.5, arguably. And stats are supposed to be a jumping off for conceptualizing your character, not the most important thing about 'em. Not to mention the whole 'how to roleplay mental stats without it meaning 'player skill must be gimped if you got a bad roll in character creation' As such I'm looking favorably at Maze Rats style stats.
Another thing I wanted to do was emphasize that rolling dice has consequences on failure as well as success, to prevent haphazard 'I sneak I search I speech' thrown out willy nilly, and that dice results stick until the situation changes. You don't roll to sneak past each guard, cuz that's called 'rolling to failure' and is a form of railroading, where the GM calls for rolls until you finally fail one. The opposite problem is 'Taking 20' where there's no consequence for failure or dilly dallying so rolls become pointless. Ideally, both are solved by the totally not butchered osr principle of 'rulings not rollings' and making time a resource. The powered-by-the-apocalypse style rolls I used in Monsterhearts where on 2d6, 0-6 makes things worse, 7-9 is mixed success, and 10+ is pure success was pretty nice and I might adopt it, but keep the d20 around for ease of stat conversion for attack rolls.
Character sheet wasn't simple enough. Skill slots, inventory slots, hp, ac, chosen traits, etc were numerous enough that it couldn't be edited onto blank online text document elegantly. Though I liked Notches, keeping track of them was also annoying (A player quickly found themselves keeping track of Notches for Fists, Daggers, Bow, Sword, and Torch in a handful of sessions) and the system encouraged fighting for the sake of fighting- especially fighting lots of weak foes. My tentative solution is to say that you only keep track of notches for ONE weapon class at a time, and to make it per HD slain rather than per enemy, and probably only for things with higher HD than you. Or maybe disregard the system and use my preferred 'secret technique' style of fighters learning cool stuff that I used in the BFRPG game. Probably that, yeah.
Too many class abilities that were too close to snoozeworthy mechanical bonuses and passive bonuses rather than abilities that made people say 'wow I can do THIS now?' Also, too forward-facing. Cavegirl mentioned using secret booklets to gradually introduce lore and character abilities and I think this could be great both to prevent character creation from being bogged down and to prevent players from obsessing over getting Cleave at level 3 and so roleplaying as a player wishing to upgrade their character rather than a character wishing to
Fighters did a bit too much damage with the 1d8 bonus auto damage from The Nightmares underneath. I think having them be the only class that gets bonuses to hit is sufficient, ala Lamentations of the Flame Princess.
One of the things I dislike most about D&D is 'scaling' and I think there was still too much of it going on. Ideally, I want players to genuinely not care if they are a few levels below another PC, and to definitely never view monsters as things that require level grinding to face. Dragons should be scary because they can smell metal and breathe fire and are hard to escape from and negotiate with, not because their maths are ten times bigger than your maths so you need to math yourself up x10 and then you can fight dragons fearlessly.
I also decided coins for XP is a great OSR idea that I just can't get to mesh up with how my games are set up with rotating character rosters. Player A comes to 5 sessions and obtains 5 oddball magic items but 0 coins, and player B goes on a solo to pick through an 'already looted' dungeon and finds a secret stash of platinum and can now gain 2 levels. Player A only has 2 hands and 1 action per round and limited creativity with those oddball items and really would just like to be level 3 because levels are a way progress is tracked and being knighted and having the favor of Duke Ekud and having a bunch of potions but still being level 1 cuz you didn't find gold in a hole rubs many the wrong way. Plus a lot of players seemed to like the 'alternate leveling systems' so a Maze Rat style where each session is worth 1xp, great success is 2xp, and success against odds thought overwhelming is 3xp would let play for the sake of play and pursuing player-set goals hold up, rather than mandatory loot-lust. Getting a fat pile of loot probably counts as great success so loot-lust is still viable, just not required.
tl;dr, I adopted the GLOG because I love its magic system, but should have thought about other things much more carefully before running away with 'em.