I remember when this system was first announced, it sounded cool and reminded me of the socket system on Diablo II. Then I saw it implemented and it was kind of bad. I guess bad isn't the right word exactly... maybe more like unnecessarily complicated. First, you have to get gear that has slots on it. Then those slots get specific elemental alignments, amounts and shapes applied to them. Then you need to hunt evoliths that fit into those slots and you might not ever get the stats you wanted in the first place and if you do get stats you want on a stone, it might not be able to fit into the slot for one reason or another.
The other issue was that the gear that could get slots was very limited anyway. We had those unique looking weapons from the Fomor HNMs in the past, but then the boosts we could give them weren't all that impressive for all that long really.
Mainly, it just seems like this system was added and abandoned almost immediately.