To expand on FrankTrollman's list:
4> Psions and Psychic Warriors have no splatbook, and most third-party sourcebooks don't add anything new for them either. Therefore, if you play one, you have to convince your DM to allow either Mind's Eye (WotC website) or If Thoughts Could Kill or Mindscapes (third-party). Preferably all of the above.
If you don't do this, you'll really be hurting.
5> The whole "psionic combat mode" thing tends to be more of a drawback than an advantage, unless you use the alternate rules in Mindscapes.
6> Psionic damage powers suck. Whitefire does about 1/3 the damage of Fireball once you get to 10th caster level, with the two the same in every other way. Part of this is the scaling issue, part is that psionics just isn't designed for area spells; it's mostly single-target stuff. On the other hand, their nondamaging stuff can often be slightly better than what a Sorcerer would get.
7> Their DC uses 1d20+ instead of 10+. Overall, randomness slightly hurts the PCs since it makes the magic even less dependable, but this is a minor flavor thing.
On the other hand, Psions get:
1> No arcane failure issues; armor up as much as you want.
2> Hold and Silence don't do squat against you, and you don't need any material components (expensive or otherwise). The lack of expensive components turns some good spells (Forcecage) into GREAT powers (Mass Cocoon).
3> 2 extra skill points per level, with skill lists that actually mean something.
4> The ability to put a lot of points into the "useful" physical stats and still have good spellcasting ability.
5> Their spell list includes an interesting mix of Wizard, Cleric, and unique powers. Some buffs (many self-only), some divinations, and a bunch of utility stuff.
6> Psicrystals are just inherently cooler than a ferret, especially if you add the Improved Psicrystal feat from Mind's Eye. Ditto for tattoos versus scrolls.
7> Access to the Psionic feats, like Speed of Thought and Inertial Armor. For a straight Psion these don't mean as much (although I highly recommend taking Speed of Thought once no matter what), but for a multiclasser it can be huge.
Anyway, the point is that Psions come off as marginally weak without outside material, but part of that is that their advantages aren't obvious ones. On the other hand, they're GREAT for multiclassing, since you can armor up and don't need to take a huge stat/skill point hit.
Psychic Warriors are already a pretty strong class as is.