3. Strong archetype classes
One of the claims I hear about B/X is that it is a great "whip out the box and play" version of D&D. The nice thing about the B/X rules is that the number of classes is very limited but they cover all of the standard archetypes of pulp fantasy.
There are only seven classes in B/X:
I find that these classes are general enough to portray nearly any of the character types of the pulp fantasy genre. While many people may find the limited number of classes to be restrictive, I find that the low number of classes forces players to use their imagination to develop their character.
While someone could view all clerics as being basically the same, through some good roleplaying and descriptive flavour, the cleric of the frog god of chaos can feel very different than the cleric of the righteous brotherhood. For example, the cleric of the frog god may decide that they would never cast a light spell because their deity prefers dank darkness and that when they are under the effects of a bless spell there skin takes on a green colour covered in warts. The cleric of the righteous brotherhood however, may always be required to take a light spell. each gives very different flavour without any specific mechanics - just roleplaying.
Fighters can be any of the warriors from fiction or pulp literature. Anything from Conan to Jason to a Samurai can be portrayed with this class - you just have to use your imagination.
Magic-users are a bit unique in B/X. I believe that the B/X rules allow Magic-users to be very diverse and fill a number of the roles of various wizards and sorcerers in literature.
Some people have an issue with the thief and how their skills are the cause of much evil in RPG design. I have vacillated on this topic and, at least for now, have settled on the side that doesn't mind thieves. In terms of archetypes however, I find the thief does fill a required role.
To me, the human classes of B/X can fill any of the archetypes with a little imagination and the right attributes. I mentioned in my last Why B/X is My Favorite post that B/X's moderate mechanical bonuses for attributes gives each character some uniqueness. If you roll a high Dexterity your character could be a quick, swashbuckling fighter. A high constitution could be used for a street tough thief. A high strength could be used to make a vengeful cleric of an intolerant deity. As will all version of D&D, a little imagination can go a long way.
I have already discussed the demi-human classes in a previous post and I will cover a few more points when I discuss reason #4 - Race = Class and meaningful level limitations