Inspired by Materia (FF7)

For some reason, the materia system from FF7 has stuck in my mind for years as one of the most interesting RPG skill systems I've every encountered.  I was quite disappointed that neither of the FF MMOs based their system on it at all.  I'd like to design a new system inspired by materia but distinctly different from it.  So I'd like to know, what do you all think are the cool/interesting/fun/compelling parts of the materia system, vs. what parts didn't work very well?

Personally I think the key good part of the system was choosing which materia to put AP into.  Also, junctioning two materia was pretty cool.  And the whole "magical crystals that grow with human energy put into them" is great lore.  On the other hand, it might have been better to have materia permanently attach to a character.  And more interaction between materia based on the level to which they had been raise would have been good.  For example, maybe a character would have to have two basic elemental materia raised to level two in order to unlock level one of the secondary element that was the combination of the first two.  Perhaps more levels would be better.  Perhaps the Enemy Skill materia could be broken up among the others, such that there were 2 or 3 learnable monster attacks associated with each materia.