A recent post at BX Blackrazor reminded me of one of the things I think is an important feature of TSR D&D, in all its forms, that was turned on its head in WotC D&D.
Clerics can wear armor, fight, and use spells (as can the Elf class in Classic). Yet the Cleric has (again, in Classic) 8 spells per level, while the M-U gets 12 spells per level.
In BX, the Cleric gets up to 5th level spells (again, so does the Elf), while the M-U gets 6th level spells. AD&D and BECMI allow up to 9th level spells for M-Us, but only 7th level spells for Clerics.
That's a bit of balance that I think is important. Clerics (and Elves) can do both fight and cast spells, but they don't get to fight as well as the Fighter (with a lower Hit Die type, and limited weaponry for the Cleric and level limits and a high XP cost for the Elf), nor have as much spell casting oomph as the Magic-User (again, with the level limit blocking Elves, and with the generally smaller list, lower ceiling, and in Classic not getting a spell at 1st level for Clerics).
That's the sort of simple and intuitive trade-off I like.