I couldn't help but add 5 more things to the "wish someone told me this" list!
5) Never eat on Las Ramblas unless you've come to sample some of Spain's legendary microwave cookery. Turn down bread at Quinze Nits- it's not worth the cost. Also, on bank holidays the Japanese Buffet on Via Laietana doubles its price.
4) "Chequenta" and "Muy Vale" are not examples of correct Spanish and may irritate the waiter, but maybe if enough people use it we can start a new trend.
3) Bicing is great, but make sure your bike locks properly or you become liable for stealing a bicycle. Also, know that Bicing bicycles are a lot like my projects after an all nighter- you never know just what you're going to get.
2) Nerd Tip: Vodafone, Orange, and Movistar are all ridiculously expensive. The only cheap phone provider is Yoigo, but "The Phone House" will try to tell you they don't sell it, or that they do sell it but only for residents of Spain. Your best bet is to grab a flyer and point to it and call them a liar.
1) When informing a prostitute you do are not interested in their services, a reason for your decision is expected.