Having done countless interviews as a prospective employer in customer service and retail environments, I support Brad's position on Jeans. It just doesn't look professional enough. Take a look at the vast majority of retail and customer service workers, and they're typically wearing black dress pants. These aren't expensive - as a matter of fact you can get a paid of black dress pants cheaper at lowes than you can buy kirkland denim jeans at Costco ($12.99 vs. $29.97), but they look FAR MORE PROFESSIONAL. A nice plain coloured Polo would be ok in my eyes for a position such as this, but as you move up in the food chain (think guest services) you'd be looking for a full button down shirt or blouse.
And for goodness' sake - MAKE SURE THEY ARE IRONED!!!
Something I look at when I do interviews though, is shoes. First of all - let me be clear - LEATHER. DRESS. SHOES. (Usually black). If you're contemplating wearing joggers, street shoes, or anything else along those lines - leave now. I was going to suggest school shoes as an example but realised most people wore joggers to school in later years. and girls - NO BALLET FLATS.
It's easy enough to drop your pants (shut up @reanimated35!!!) or shirt at the dry cleaners and pay $15 to have them cleaned and pressed ready for your interview, but everyone always wears the same old black leather shoes they wore to school \ their fast food job etc. Filthy, scuffed, unpolished shoes will be a quicker 'no' from me than Jeans - regardless of the rest of the presentation.
And @AllegroCrab - (to answer the last part of your post) this is why - When it comes to retail, and customer service, the first impression is usually the ONLY opportunity to capture, or lose the customer. A person who shows up in dirty, scuffed or unpolished shoes shows that they didn't put any real effort into impressing me. Same with Jeans, or a t-shirt. And if that person doesn't think I'm worth putting in the extra effort to polish their shoes, and iron a pair of $13 dress pants - how much effort are they going to put into presenting well to my customers? to serving my customers? to representing my brand?
If there are 50 positions, and they're pressed for time, they may lower their standards to get people in the door because they need them, but if there are 10 positions, and they've got 100+ candidates, and they've got a few weeks to get you onboard - rest assured they will be looking for the quick wins on who to eliminate.