In Basic Fantasy it is simply put the original Red Box mixed with D20 which is a truly great thing.
It only has four classes Fighter, Cleric, Thief. and Magic-User.
It only has four races Human, Elf, Dwarf, and Halflings with class restrictions but no level limits.
The only real D20ish thing is Ascending Armor Class other than that it is the Basic Game that many of us love with better options.
The rules are clean and perfect with many additional classes and races available for download on the www.basicfantasy.org site though I would most likely only allow the four if I were running Basic Fantasy Game.
Do you really need all those classes out there? Can't you just play a fighter like a ranger or barbarian and forget the details? Have your thief hire out as an Assassin? Though I must admit the Assassin supplement was written by one of the greatest gaming minds in the world and is sheer perfection possibly one of the greatest gaming supplements ever written in the universe to put it mildly.
The rules are just perfect. They work. The layout is clear and concise and the artwork and the editing is fantastic and very old school in style.
Now the free modules are where it is at! The modules are well done and very "Basic" in feel. Yes! I mean that cool about to go on an adventure feeling. They play out just like something you would have seen back in the 80s. All of them that I have read are just great old school romps!
Sadly, years ago when I ran a Basic Fantasy game the modern rule based D20 gamers acting if they were being forced to eat feces while the real gamers had a blast. Next time I run a BFRPG I will use the modules.