Over on Critical Hits, there's a post about people's attitudes towards getting science in their fantasy, with the specific example of psionics in D&D. Now, I have nothing against SF; despite my focus so far on medieval fantasy, I've also been working on a '50s-style rocket patrol RPG and a few other SFish game materials. I don't even have a complete aversion to SF, or psionics specifically, in D&D in general. However, I've got a couple complaints about D&D psionics. One is the clunkiness of the mechanics: I'd prefer something quick and simple. But more important, D&D psionics as presented spoils the feel of fantasy.
It's not that sword & sorcery or epic fantasy can't have psionics. Clark Ashton Smith inserted serpent men with psychic hypnotic powers into a couple of his Hyperborean and Atlantean fantasies, for example. Other fantasies have the occasional character with an innate power that, for all intents and purposes, could be psionics. However, different settings have different feels, and how psionics are presented in comparison with magic has a critical effect on the feel of the fantasy. The usual presentation of psionics is as a spell-point alternative to regular magic, but slightly stronger on the low level and slightly weaker on the high level, and with no restrictions like dispel magic. Let's face it, it feels like something made for power-gaming.
Of course, psionics are presented as an optional system. However, they are intertwined with a few monsters and magic items, making it hard to completely disentangle psionics. A much simpler psionic system would be easier to tailor to individual settings and much easier to tone down in power level.