I would add a tow strap. Sometimes just a short tug from another vehicle is all it takes to get one out of a bad spot.

Taking the larger view of keeping your vehicle moving (not just unstuck), beside the tow strap, shovel, and gloves, there are a few other things you might consider adding. I keep a small flashlight or headlamp stashed in the vehicle. Also a very small tool kit (adjustable wrench, pliers, 6-in-1 screwdriver), a few hose clamps of various sizes, and a small roll of duct tape. A selection of fuses can be handy. Also a quart of oil. There are compact tire patch kits that are sometimes usefull. Most of this stuff can be stashed in a small vehicle.

In part it depends on what kind of area you are traveling in. You aren't going to make major repairs with this stuff, but sometimes a few little things can help you limp down the road to where help is available. This can be important if you are out on seldom traveled roads. Other places a cell phone and credit card might be the best roadside survival tools!

EDIT: I forgot to add one obvious item...Jumper Cables.