Here are a few thoughts regarding Halloween costumes. I think that most of these apply in all costume situations, mind you.
First, store-bought costumes usually look cheezy. This is especially true of Batman costumes. Oh, and anything with fake muscles looks kinda dumb.
There are exceptions. For example, I saw an Imperial stormtrooper costume that was great. It was obviously store-bought because it consisted of molded plastic. Still, it was terrific.
Second, store-bought costumes are boring. Why? Because anybody can buy a costume and put it on. There's no creativity involved. That's why even when the costume looks awesome (such as the aforementioned stormtrooper getup), they don't particularly impress me. People tend to appreciate costumes that involved some more personal effort.
Which leads to my third point. Store-bought costumes can still look good if we invest some personal effort into them. If you're going to dress up as the Joker, for example, you can add the kind of facial make-up used in "The Dark Knight." Or if you want to dress up as Superman, carry a fake lead box with some green rocks inside.
The fourth point: Try to be creative. I'm always disappointed when I see women dress up as witches and then try to compete in a costume contest. You're not going to win, folks, not unless the winners are selected based on popularity. Dressing up as a witch takes no imagination.
Instead, why not pick some non-obvious choices? Dress up as Captain Stubing, for example, or as The Six Million Dollar Man. Dress up as Alan Scott instead of Hal Jordan (though Hal Jordan would still be an awesome choice, given that he's not as obvious as Superman or Batman).