Kabul is my current favorite restaurant in Philadelphia, and probably the best Afghan restaurant I've ever been to. It's a popular spot that fills up quickly on weekend evenings. The atmosphere is warm and inviting, even romantic - but what keeps me coming back is the food.
From the appetizer list, try the Bulanee Gandana and Bulanee Kadu - fried turnovers filled with scallions and herbs, and delicately spiced pumpkin, respectively. They remind me of fried wontons. The complimentary salad is rather unpretentious - iceberg lettuce and sliced cucumber, with a few carrot shavings tossed in. The dressing makes it special, though. It's a zingy, tangy, yogurt-based concoction that tastes of garlic, mint, and I think cilantro... very tasty. (Vegans, be sure to request no dressing or some sort of alternative.)
The menu boasts many vegetarian entrée options. One favorite is the Norenge Palaw - chunks of eggplant simmered with tomatoes and spices, covered by a mound of sweet, cardamom-scented saffron rice, and topped with almonds, pistachios, and slivers of candied orange zest. (Be sure to specify "vegetarian", as they make this with lamb as well.) Another favorite is the Chalaw Lobya - kidney beans simmered with onions in a wonderfully spiced tomato sauce, and served with a plate of deliciously spiced white or brown basmati rice. The beans are topped with a touch of yogurt and a dash of green powder, which I think is finely ground mint leaves. (Many things are topped with or accompanied by yogurt here; I'm sure this can easily be omitted upon request.) I also love the Chalaw Sabzi - spinach cooked with kidney beans and spices, with a touch of tomato, also served with white or brown basmati rice. Another winning dish is Facillya - green beans cooked with onions in a tomato sauce. This dish is more subtly spiced than some of the others, but it's a bit spicier in heat level. Not hot, mind you; just enough to make it interesting. Simple and flavorful. This is home-style cooking at its best. Luckily they also bring you Afghan bread, kind of the Middle Eastern version of focaccia; it's perfect to sop up all that delicious sauce. And if you can't choose from the lovely selection, don't fret - you can order side dishes of most things for $4!
If you have room for dessert, and dairy is doable, do try the Sheer-E-Yakh - vanilla ice cream topped with almonds, pistachios, and a pink rosewater-flavored syrup. It's divine.