In my humble oppinion, the most important differences between Spanish dialects (at least because it can be very embarassing if you forget about them), is on swear words. Curiously, there are some common verbs which are totally neutral in some places and offensive in others.

For example, in Spain we say coger for the English take. So, I took the bus would become Cogí el autobús. Problem is that in most of South America, coger means fuck. And well, it is not something you can do with such happiness!

That's the reason that most satellite TV channels who broadcast to Spanish regions have interesting style manuals with lists of words which change meaning dangerously. Christian prayers were also recently (ten years or so, I seem to remember) unified so that they were perfectly correct and the same everywhere.