4. Race = Class and meaningful level limitations
I love the B/X race as class. To tie this to my last Why B/X is My Favorite post, to me race as class represents the purest form of the archetype system. It make races special. Playing an elf, dwarf or halfling should be more than playing a human with a special ability.
While this is also linked to a future post about:
8. A tight power scale with a max of 14th level.
I find that the lower level limits of demi-humans and the attribute requirements (assuming one is doing 3d6 in order) has the desired effect of keeping demi-humans rarer than humans. I prefer campaigns that are human dominated.
The first 3 books of OD&D play the same as B/X by default as the demi-human races were restricted to a single class - except for elves which are completely different.
While Mentzer's BECM also has race as class, it was very watered down by the expanded range of levels (1-36) and the expanded combat capabilities.
The separate race and classes of other editions just can't compare to me.