3.0 Psionics only have two base classes. Psion and Psychic Warrior. Psions select one primary discipline, and use the associated ability score as key ability score for bonus power points. They gain ponus PP for that. Psychic Warriors don't gain any bonus PP.
You need decent scores in all 6 ability scores to be an effective manifester, since every discipline has its own key ability score. So the save against your psychoportation power is based on dexterity, but your telepathy powers need charisma. You'd need 19 in all stats to have free choice of level 9 power.
Damage dealing powers are practically useless, as they often use d4s and almost never scale. Don't try to be a psionic artillery.
Several feats need a power point reserve. Once you drop below that, you cannot use it any more. Others require you to spend PP (they are like additional powers, since you have so few of them) There is no psionic focus.
Powers that are augmentable in 3.5 are often several different powers in 3.0, forming power chains. So while you only need Astral Construct in 3.5, there are 9 different Astral construct spells in 3.0. Also, concussion blast is something like lesser concussion, concussion, greater consussion, mass concussion, true concussion. The only difference is more damage dice.
There is still psionic combat, and especially the mind blast combat mode is insane. Also, it's a pain to determine the dc based on what you use and what your enemy uses
The PrC's had no "+1 manifester level" part, but instead had extra power lists (so you start again at zero)
On top of the difficulties determining the DC, with psionic combat and multiple ability dependency (as you need all 6 ability scores, one for each discipline), you had to roll 1d20 and add the other stuff, instead of just using 10 + stuff.
On the other hand, 3.5 has new psionic races.
It has 4 instead of 2 classes (one of the new ones is a complete overhaul of one of the old PrC's, the Soulknive, but the other is completely new - The Wilder, which has only very few different powers, but can boost them beyond the usual limit, has a better HD, a better BAB, and other goodness).
Powers still don't scale automatically (so while a level 7 wizard has a 7d6 fireball, your energy ball stays 5d6 unless you do something about it), but you can augment many powers - you pay an additional cost, and they become more powerful. This doesn't just include scaling for damage dealing powers, but it's also used to combine whole power chains into single powers. So instead of 9 AC spells, you just have astral construct, and can augment it. You have only one concussion blast spell, and can increase damage or get additional targets with more PP. Dominate is concentration and for humanoids only, but you can get more monster types or prolong its duration with more pp. Note: You cannot spend more PP than your manifester level, so you cannot blow all your pp into one big energy ball or something.
Most of the old power-like feats are now real powers, as are the (heavily changed) psionic combat modes (they are often completely different, only the name remains). Psionic combat is no more.
You now have a psionic focus. With a DC 20 concentration check, you can focus yourself. It takes a full-round (or move-equivalent with the right feat), and you must have at least 1 PP remaining. Some psionic feats require you to be focussed, and others require you to spend the focus in order to activate them. Metapsionic feats are among them (but they don't "cost" as much), as are feats like psionic weapon. In practice, this starts as a one-per-combat thing, as it takes a full-round action and a difficult check, but later you can refocus in the middle of the combat quite easily, taking only a move action and with higher rate of success due to a better concentration bonus.
All in all, just stay away from it, 3.5 expanded psionics rules are vastly superior. Just don't use the psionics are different rules unless psionics are a major theme in your campaign. And you might want to cut down some of the augmentations down to size (especially the energy powers, which increase the DC by one with every pp invested, instead of +1dc for every other pp)
