First of all, here's how I understand this works.
When you hit an enemy (with a weapon attack, whatever that exactly is, we'll come back to that) the attacked creature loses its dexterity bonus (how does this interact with uncanny dodge?) to this attack (no effect unless you have sneak attack) and to every further attack (from you) until the end of your next round.
Benefit: The magus can expend 1 point from his arcane pool as an immediate action after hitting a target with a weapon attack, allowing him to anticipate his opponent’s defenses. The target is denied its Dexterity bonus against the magus’s attacks until the end of the magus’s next turn
What are weapon attacks?
Certainly manufactured weapons such as daggers, bows etc, likely unarmed strikes but what about natural weapons?What about ray attacks? You can take weapon focus in them, so they should be weapons?
What about fiery shuriken, we play with being able to take weapon focus shuriken to get the +1 on those attacks so they're weapons. Maybe?
What about a river whip? It functions like a whip except you make touch attacks.
What about other spells, particularly the weapon like spells such as acid arrows, flame arrows, mages' swords or spiritual weapons?
I suppose the issue with spiritual weapon and mages' sword is that you're not actually the one wielding it.
I kind of assumed it would be more restrictive than that.