I look for good fit (so the damn skirt stays in place), a sensible fabric, and a nice active cut. If I need a slit to walk, it's cut too narrow. If it needs a crinoline to hang right, it's too full. Most skirts are too narrow to bike in easily - they're fine if you have a step through frame and are not going far. If you have a diamond frame or are going more than 3-4 miles, you'll really want something you can move in.

Kilt style wrap skirts seem like they'd work best. Good freedom of movement, and they only need to fit properly at the waist. Fixing fit issues is also pretty easy, even if you can't sew very well. I've been having a hard time finding 'em tho