Another thought on psionics, compared to other types of magic: Psionics is more straightforward - that is - it's good at what it does, but it doesn't have all the niche uses and tricks of magic.
The first way this show up is the different types of powers - they're more balanced against each-other. In arcane/divine spellcasting, some types are weak (like direct damage), and some types are strong (like battlefield control). If you've played long enough with core spellcasting, you may come to accept this as normal. Then here comes psionics, where direct damage has been buffed up to spec and some other types have been weakened or even dropped entirely. If you're used to core magic, you might think this is an imbalance - but compare the actual utility.
The second thing is that powers, being straightforward, are often easier to use to their full potential, whereas spells may have hidden power that takes an experienced player to find. For instance, summoning. Many people say that Astral Construct is broken because astral constructs make better tanks than many summoned monsters (although not compared to SNA). However, can Astral Construct summon a creature with a ton of potent spell-like abilities, or spellcasting of it's own that's better than the caster who summoned it? Of course not - that's the hidden power of Summon Monster. But if you just look at the surface, you won't see that.
The third factor is the experienced player law. The EP Law is simply this: A more experienced player can make an effectively stronger character, regardless of class. This is a fundamental principal in all games. Therefore, the non-core corollary: Classes that require more experience (from being non-core and/or complex), will seem more powerful, because they attract more-experienced players. If psionics was core and Vancian magic was only found in the "Expanded Arcana Handbook", then I bet people would be complaining about how broken Wizards and Sorcerers were.
And final factor: The lone psion. If you run a totally non-psionic campaign, but have one PC be a psion, they'll be more powerful, just by virtue of being unique. Imagine a campaign where Druids didn't exist and nobody had heard of them, but one PC was a Druid. They'd be incredibly powerful - nobody would realize that they could turn into animals, so they'd usually be able to escape any captivity or spy on people totally undetected. Their spells would take people by surprise, as their foes would have no defense against Druid-only things like entangling vines. That's the same situation that happens with many Psions, which leads to DMs that think psionics is broken.
I'm not saying you should revamp the whole setting, just specify that there are a few other psions around, that the more knowledgable people know what psionics is and how to recognize a user of it, and that the BBEG could potentially hire (or be) a psion, or at least have psionic items like tattoos. Also, the "Psionics are Different" option is only a good idea if you're an experienced DM and very familiar with psionics. Otherwise, it can lead to all kinds of problems.
