Ah, a troll lord, I like it. I like the way the ability naturally goes on your creatures or your opponent's (you might even want to make your opponent's defender attack and now you can).

I wonder if it should be able to untap opponents creatures (since most creatures you want to make attack have tap abilities) but agree it would be hard to word as elegantly.

It's really hard to make a tap-ability creature attack. Provoke couldn't make one block. You'd have to have something like "Untap target creature. Its activated abilities can't be activated this turn. It attacks this turn if able and may attack as though it didn't have defender." So yeah, I don't think that's worth doing.

Good point. "Activated abilities can’t be activated this turn" would go well with untapping your own creatures too (so they can't abuse the untap, but do get to attack), and "can attack as though it were untapped" is possible but dangerous territory, but there's no elegant way of letting it work for opponent but not you, so I agree you were probably right with the initial design.