I have noticed this same problem. It's very frustrating. It seems as though you have to wait until the animation from the first ability is complete before trying to use another.
One place this is actually helpful is when trying to break doors or chests. I noticed that if you, for example, use a wizard's [Staff of Magus] ability on the door/chest... wait until just as the projectile is released from the character, and then mash the Staff of Magus button a bunch more sometimes I can get it to cast as many as 5 times in rapid succession. This doesn't seem to work in actual combat because it consumes all the AP - but out of combat it's kind of funny.
I'd sure like to see it fixed, though...
