One of the bigger discontinuities between LBB-only OD&D and post-Greyhawk OD&D is the way ability scores are handled. In the former, the prime requisites -- Strength, Intelligence, Wisdom -- provide an experience point bonus and only to the class with which they're associated. Meanwhile, Constitution, Dexterity, and Charisma all provide some mechanical benefit that applies to every character, regardless of class. Consequently, there's neither a mechanical advantage nor disadvantage for, say, a fighter to have a low Intelligence or a high Wisdom.
Supplement I changes this dynamic by providing modifiers for Strength and Intelligence (but not Wisdom, curiously), so as to make high scores in them more valuable for fighting men and magic-users respectively. In addition, the introduction of the thief, whose prime requisite is Dexterity, also had the unintended side effect of making most thieves very good at the use of missile weapons.
When I began my Dwimmermount campaign, I wanted to go for as "pure" an OD&D experience as possible, so we initially used only the rules in the LBBs. That rather quickly changed, because, like variable weapon damage, a wider range of ability score modifiers is one of those aspects of later editions of Dungeons & Dragons that everyone expects to be there. So, we used Supplement I in a quasi-AD&D fashion (e.g. granting to hit and damage bonuses for high Strength to all characters, not just fighting men).
Lately, I've been pondering the idea of two sets of ability score modifiers. One set that's for all classes and one set that's only for members of a certain class. What if, say, bonuses to hit with melee weapons was available to characters of any class with a high enough Strength score, but bonus to damage was only available to fighters? There's precedent for this even in OD&D, where high or low Intelligence has consequences for a magic-user above and beyond an XP modifier but for no other class. My intention here is to restore a little of the unique association a prime requisite has to its class while at the same time providing benefits and drawbacks to all classes for their ability scores.
This is definitely a great deviation from LBB-only OD&D, but I'm OK with that. After years of experimenting, I find I'm happiest playing D&D 0.75 and this is in that vein, I think. But I'm curious to hear what others think about this, at least in the abstract. If people want, I can make another post later where I lay out the full extent of what I'm imagining and we can talk more specifically about that.