2. Unified bonuses & penalties due to attributes
Come on... the stat bonuses in AD&D (1E and 2E) are ridiculous. And percentile strength? Who thought that up?
Now I have no problem with the OD&D and Holmes versions giving no real in game mechanical bonus or penalty for stats outside of those for experience points. Philotomy's OD&D Musings has a great discussion about OD&D's handling ability bonuses, and its philosophy of bonuses, in general.
Moldvay/Cook uses a unified set of attribute modifiers based on:
9-12: no bonus
Off the top of my head, I can see a handful of reasons why the way that attribute bonuses are handled in Moldvay/Cook is attractive:
1. Moderate mechanical bonuses for extraordinary attributes gives each character some uniqueness
2. It does a really good job of following the bell curve of 3d6
3. It is really simple to remember
4. It provides an in-game mechanic for each stat making sure there is no such thing as a "dump stat"
5. Fits in nicely with the attribute requirements of demi-humans to focus on the archetype
And while I can appreciate the unified attribute modifiers in later additions, there are two things I do not like about the way they handle them: they do not follow the bell curve and seeing a character with a strength of 22 is just plain jarring.