The Harlequins two battle traits are "Lords of Leaping", which lets them do the 40k retreat and shoot thing since they all fly, and "Consummate Performance", which lets them change with the different 'Acts' of the battle. Lords of Leaping is contingent upon being close to a Harlequin Hero, but is pretty potent I would imagine since they've got all that short range shooting. I also wanted Harlequins to be mobile in weird ways, so it should work with that. Consummate Performance I am really pleased with, I tried to think of things that would be both flavorful and useful during different parts of the battle. In the first act, their characters are protected from alpha strikes as they introduce the cast of players. Then, in act 2, they can run and shoot/charge, because who doesn't love that? Finally, in act 3, they count casualties as double for battleshock so that they can hope to break some stubborn holdouts. You get to control the progression of the acts, so you can stretch it out over the battle, or try to move quickly to your favorite act.
The Artefacts of Mirth are both pretty conventional, although there are some wild Artefacts in there. My favorite is the 'Script of the Laughing God'. It lets the bearer choose which Act they're on as they skip around in the script.
I generally try to spread out Command Traits between making the general awesome, cool tactical abilities, and buffing nearby units. While all three made it in here, Harlequins are somewhat of primadonnas and so there are a lot more abilities than usual in here that just make the general awesome.
Anyhow, the full battletome is now live on the 40k warscrolls page. Enjoy!