Anyone curious what those looked like in earlier versions of D&D? No? Too bad, I had to look at this and so do you.
With that out of the way, thought eaters are very underwhelming for their threatening appearance. They're relatively low-level, with three HD (aberration type, because we can't have nice things), small size, and limblessness.
Their ability scores are a mixed bag. +8 dexterity, +2 strength, +2 wisdom and +4 charisma are all neat, but +0 constitution and -4 intelligence aren't. +2 natural armor is okayish.
As for their natural weapons... thought eaters don't have the bite, or claws, or really anything you'd expect of them. Instead, they have a natural touch attack that deals six power points of damage or, against nonpsionic creatures, a single point of intelligence. It shouldn't be hard to see how outright terrible that is.
Thought eaters also get some PLAs, which are somewhat better. At-will Psionic Daze can be used to lock down low-level humanoids, at-will Precognition is essentially a 1/encounter boost where it's needed most, and at-will Detect Psionics is useful for obvious reasons. The other abilities (3/day Thought Shield and at-will Distract) are less notable.
Finally, there's the ability that makes the thought eater interesting again: Ethereal Jaunt. The eater spends most of its time on the ethereal plane, and can shift to the material as a move action. As a free action, it can then shift back. At ECL 3, this ability is huge, and lets you bypass essentially all physical barriers, scout all encounters, ambush foes at key moments, and escape combat whenever needed.
One important drawback: the thought eater can't actually stay on the material plane for long, and dies if it spends ten consecutive rounds there. This is moderately annoying most of the time, but becomes outright dangerous if you get knocked out.
For the asterisk, I'll assume that both Ethereal Jaunt and the inability to stay on the material plane are removed (mostly because the second without the first means a very quick death). Without those traits, the thought eater is a very unimpressive creature, whose lack of thumbs, viable natural attacks, and useful traits condemn it to -0* LA purgatory.