Personally, I dislike the idea of shards because I don't like the "rare card" idea at all. I believe that every single card should be available in the bazzar (even Nymphs).
I also may like the idea of neutral cards which can fit into any color, however their neutral cost is too small in rainbow decks which produce 12 quanta with a single Nova, or 3 quanta per turn with a single Quantum Pillar/Tower.
Their effect is also quite OP in certain shards (SoSa).
I am also against the fact that permanent Shards are stackable (SoG, SoV) and can be protected by a single Enchant/Protect Artifact spell.