I always bring an main dish because I figure most people prefer the less-expensive or room-temperature options. I don't go to a lot of potlucks, so I don't mind a little extra expense or time. My feeling is that if someone else is hosting, I can help this way.
Meatballs in a crock pot
Chicken with lemon sauce and rice *
Sliced roast beef with mushrooms and rice
Oven-BBQ chicken legs
Cooked turkey breast, sliced
Grilled vegetables
Macaroni with string beans
Cheese Ravioli or Tortellini with sauce
I sometimes choose Minestrone or Chicken soups, which I bring in the crock pot, too. If it needs to be heated quickly, just microwave it for a few minutes then put it in the crock and it'll be ready with the rest of the dishes.
* Chicken with Lemon Sauce and Rice: easy recipe and because the chicken is cut small/pounded thin, it goes a long way. I usually make 4 pounds or so - leftovers freeze well. Pound strips of boneless chicken to a 1/4" thickness. Dredge in flour seasoned with salt, pepper, 1 tsp. powdered thyme and a little paprika (for color.) Saute lightly in a little butter and olive oil, then put into a foil baking pan. Add a quart of chicken broth, 1/4 cup chopped fresh parsley, juice of 3 lemons and the hulled-out lemon peels to the pan. Cover and cook at 350°F for 30 minutes. I usually make 1 cup of (uncooked) white rice with chicken broth, parsley and 1/4 cup lemon juice. I cook the rice separately so that it doesn't soak up all the pan juices. Before I put it out, I remove the foil and the lemon hulls, top with a little fresh parsley and some thin slices of fresh lemon.
I also bring clean, empty take out containers that I've saved, so people can take leftovers home with them without having to return a dish.
(My cabinet's a little full of those right now; I'm hoping someone gives a potluck soon so I can clear out my stock. I think the last potluck was in May.)