My three favorite things about ACKS proficiencies are:
- These are really easy to understand due to brevity and limited choices. The explanations of all proficiencies takes up less than eight pages, and, in actual use, no player would have to read all of that, since every character class has access to only a fraction of the proficiencies. This trimming down of choice means that 1) choices become meaningful again and 2) a newbie can easily use the system without feeling that they are missing something or at a disadvantage to veterans of the system who are familiar with the esoteric ins and outs of the system (what some gamers call "system mastery").
- The number of proficiencies a character can "take" is also very limited. At maximum, a 1st level character can have six proficiencies, and most characters (those without above-average Intelligence scores) will only have three; on top of that, one of those proficiencies is automatically Adventuring, which is just ACKS' way of formalizing the assumption that every Old School game I've ever heard of already has: the PCs are competent in the basics of adventuring. At the pinacle of a PC's career, the maximum number of proficiencies a PC could have is 13, and that's in the unlikely case that the PC is a Fighter, Assassin, Dwarf, Elf or Explorer with 18 INT.
- The proficiencies don't require a "build" mentality. They don't need to be carefully chosen to interact with each other for maximal benefit; in fact, most of them can't, and when they can, it's spelled out for you. They all have both concrete, in-game explanations and mechanical consequences. A player who wants to build characters and a player who is just interested in the narrative growth of their character should both be equally comfortable using these.
I'll try to write more on this at a later date; I'm rushed for time right now, but this needed to be said, I think.