Several times over the last year, the topic of Psionics has come up in the OSR community.
I will make no bones about this: I am a huge critic of the 0e and AD&D approach to psionics. I think the approach to creating psionic characters, as presented by Tim Kask and Gary Gygax in 0e and AD&D, was flawed and unsound. On the other hand, I think psionics should play a more important role in D&D.
James Mal at Grognardia has posted several times regarding a reworked psionics system. He has given that a great deal of thought, and has tried to streamline and clarify the original rules. But the problem with simply re-working the psionics system is that you never deal with the fundamental flaw of psionics in D&D. Psionics in 0e and AD&D is employed as a subsystem, rather than using the tools that are already available in the core character creation rules. As such, it merely boiler-plates something that employs none of the mechanics that players are familiar with, and as a consequence, makes psionics rare and difficult to implement in the game.
The possession of psionics skills occurs rarely, and then by happenstance. Regardless of whether you use the 0e or AD&D rules for psionics, you have a (generally) very low chance of possessing psionic abilities. For example, in AD&D, in order to have the potential for psionics, you must have an Intelligence, Wisdom or Charisma of at least 16. And if only one of those attributes is a 16, your chance is one in a hundred of having psionics.
Compare this to, say, a magic user. All you need to be a magic user, and use spells (many of which are similar to the psionic powers) is to have an Intelligence of 9.
In a way, 0e and AD&D try to use the existing ability stats to determine psionic potential. In 0e, if you have an Intelligence, Wisdom or Charisma of at least 15, you have a 10% chance of having psionics. Period. Thus, your original ability scores do affect whether you will have psionics, but to a limited extent. The problem is, if you have psionics, your psionic ability score is completely unrelated to your standard 6 ability scores. Instead of using one of your ability scores, you roll a d100 to determine your base psionic ability. Whatever you roll on the d100 is your base psychic potential. That's right, this ability does not follow the standard 3d6 ability score determination.
I feel that psionicists should have been more accessible as a class, rather than being superimposed and boiler-plated upon the existing classes. However, doing so would necessitate a different treatment of psionic abilities, to align their use more closely with the vancian spell system that is employed for the cleric and magic-user classes.