Faking the appearance of an incorpeal undead creature.
Infringing on illusion spells.
...
But yeah, I expected it wouldn't have as much functionality as it did in the tabletop
Which is the only normal thing to do here.
First, you can't "infringe" on other spells-there is more than one way to peel an apple, and you can mimic a will o' wisp or something like it because you made a floating light, or because you made an illusion of a will o' wisp.
Also, if you could make floating lights with minor illusion we've entered into the philosophical point of "what is light?", so suffice it to say that he's right in general that there are some uses of the spell.
More generally, I would love to see them spend more time on writing these kind of interactions into the game. YMMV, but thinking of clever cantrip uses is part of what makes wizards/sorcerer's fun on the tabletop, I'd like to see a nod to that part of DnD.
