Since I brought it up, I'll take this question.
2e psionics worked a little like a cross between 3.5 psionics and a warlock. With a bit of mana magic thrown in.
So. You had psionic strength points, based off of your level, your Wis, Int, and Con (Wis was most important). You used those PsSP to initiate the various psionic powers you had. Powers came in two tiers, sciences (the powerful stuff) and devotions (less powerful/more limited).
However, there were no spell level equivalents. You had a primary discipline (out of clairsentience, psychoportation, psychometabolism, psychokinesis, and telepathy - the sixth discipline, metapsionics, could only be entered later). At 1st level, you only have access to that one discipline; as your level goes up, you get more powers and access to more disciplines.
To activate a power, you made a die roll. Each power was tied to Wis, Int, or Con, and had a bonus or penalty to the die roll to compensate for how difficult it was. If you rolled that or below on the d20, you manifest the power. If not, you lose half the PSP cost and nothing happens. If you roll a 20, you REALLY blow the power, and it backfires in an interesting way. If you roll a 1, you get superior effects.
The trick was that most powers didn't have saving throws if they were targeted against opponents. This included the ENTIRE telepathy discipline. In telepathy, you first had to use the contact power against an enemy's mind, and the worst that person could do is apply a -2 to your roll. If you succeed, you can then activate any telepathic power you know against them and they don't get a saving throw. This includes things like dominate. The only way to keep a psionicist out was if you were also a psionicist. Not even wild talents were safe, unless they happened to roll a psionic defense mode.
That said, yes, psionics were entirely playable. The easiest way is just to limit access to the telepathy discipline. There are other great powers, pretty much all disciplines have them. But they tend to have both large PSP costs and difficult rolls. I actually had a ranger wild talent that had disintegrate, but I could never make it come off when I needed it. And in the process you take your PSP's down to 0, meaning you're wandering around with d6 hit points and a crossbow until you can meditate and get some back.
And even if you add telepathy, all you need to do to scare the bejeezus out of your players is bring in the illithids. The scariest published adventure I've ever read was Night Below, where the PC's have to infiltrate illithid and aboleth cities deep in the Underdark. I mandated that at least half of the PC group be at least multiclass psionicists before we started that one, and it was still the highest body count adventure I've ever run. We started getting into Dark Sun character trees, it was so bad.
