5. Allow customization -- the more, the better. When the Stanford band is playing at sporting events, it wears a minimal uniform, but the musicians customize this considerably and nobody gets sent home for having something missing. When they're out and about, just about anything goes. It feels good to play with an avatar you've chosen and customized for yourself, particularly if you're allowed to get crazy with it.
6. Reward playfulness. The Stanford band's unofficial motto is, "music is meant to be played, not heard." In other words, the musicians' pleasure of performing outranks the listeners' pleasure of hearing, and the LSJUMB does all kinds of things to please itself.
The merits of that approach are debatable -- what works for a college band playing for free doesn't work for professional band playing for money -- but it is absolutely true of video games. They're meant to be played for the player's own enjoyment.
If you want to create joy, reward playful experimentation and zany behavior. If the player gets weird, don't punish it, encourage it and get weird right back.
7. You can't sell joy. Joy is not something you can monetize and persuade people to pay for in microtransactions. To make a joyful game you must do it freely from the heart, from a generosity of spirit.
Sell the software, sell subscriptions, or however you earn money from your game -- but don't think about money when you're thinking about joy; you'll only get some horrible saccharine caricature of the real thing. The Stanford band brings joy with it whether its members being paid or not.
Music is an essential feature of joyful games, I think, because few things touch our souls so easily and so directly. Personally, I find Guitar Hero and Dance Dance Revolution put a little too much stress on achievement and not enough on sheer exuberance, and to be honest, playing DDR feels more like marching than like dancing to me. But maybe that's because I'm too uncoordinated to play them well. Both, along with PaRappa the Rapper, were revolutionary in the way they integrated music and play.
I don't mean to say that the game has to be about music, though, only that music adds the vibrancy and color that joyfulness needs. Nor does a game have to be joyous all the way through -- like the movie Shrek, which was deeply joyous at the end, there can be a lot of other excitement before it gets there. But when the time comes, let loose! If you don't care for the Stanford band's approach, listen to Three Dog Night singing "Joy to the World" (no, not the Christmas carol), or Mozart's "Marriage of Figaro" overture.
This isn't meant to be a put-down of other kinds of games. I would just like to see more games that leave me with a smile on my face. Don't march, dance!