In Washington, DC, if you take the metro (subway), we have unwritten rules about how to properly ride an escalator.  If you don't want to walk down or walk up the escalator, you can stand, but make sure you stand on the right hand side of the escalator.  This allows those who want to walk up or down the escalator to do so on the left side of the same escalator.  If you stand on the left, especially during rush hour, you will be pegged as an obvious tourist (at the least) and yelled at (at the worst).

Also, always allow people to exit the metro trains before you try to go in; people will definitely push their way past you, sometimes roughly, if you are rude enough to try to enter the metro car while people are exiting. 

Don't believe a Mexican that tells you that a food "no pica" ("it is not spicy"), it is 90% possible that is a trap! (To be fair, our *mean* tolerance to chilli is above in comparison to many (but no every) countries so they do not intend to lie; I say *mean* because my tolerance is pretty low so I always avoid spicy food hahaha).

There are many many people that are very kind and will always try their best to help you and make you feel at home, but a golden rule (at least from my personal perspective) is: do not trust to much a person at the beginning, because there are also people that love to take advantage of others (that's one thing that I really really dislike from here), so a collorary is try to not go alone (or late), or do not leave your belongings alone, and so on. These rules may not apply in the touristic areas where there is much more vigilance(?) and the people are more respectful and more concious about giving an excellent service to tourists (but be aware that many of them will expect a tip, so if you don't take the initiative or even refuse them to gove one they could feel ver offended).

And again: be careful about las salsas picantes that you put on your tacos! :)