One of the best things about Barcelona, is that almost anything goes. There are very few places that require a dress code and unless you're wearing flip flops and coming straight from the beach, Barcelona's restaurants are for the most part, extremely unpretentious, unassuming and very inviting. Whether you're fine dining or hitting up greasy spoon tapas in a downtown neighborhood locale, casually elegant is usually about as dressy as it gets. That being said, Barcelona is a city renowned for its culinary innovation, avant-garde cuisine and Michelin Star restaurants. The Adria brothers are amongst many famous chefs who have made Barcelona the foodie hub that it has become today. So if you're looking to avoid some of the higher-end restaurants and stick to simple, tasty and more casual dining, there are several great places throughout the city where you can get quality food at reasonable prices in a relaxing setting.
The Old City in particular lends itself well to casually trendy and extremely tasty locales. The Gothic Quarter, Born and Raval are all rustic (the Born a bit chicer) Barrios in Ciutat Vella, with a mix of touristy and off the beaten path places to dine with a less upscale flare.