I was really excited yesterday to find that the D&DI was releasing a new class with the character builder update: the Psion! I was quite surprised to see that the psion uses a new system for their powers, I think it should work well, but I noticed something.
The way it works is that it eliminates encounter powers, but you not only get to gain new at-will attacks, but you can augment them with power points (which you start with two of, and gain more every few levels). The problem is that if you try to multiclass into another class (or try to multiclass into psion) you don't have any encounter powers to trade for powers from your new class.
I suppose you could lose your new at-will and the added power points in exchange for your multiclass encounter powers, but it seems like it would be weakening your character or at the very least, making things very complicated.