6. Not learning when to use articles (un, una, el, la, los, las): Foreigners learning English often have a hard time knowing when to use or not use "a," "an" and "the," and it's the same for English speakers trying to learn Spanish. Using them incorrectly usually won't keep you from being understood, but it will mark you as someone who's awkward with the language.
7. Not learning the subjunctive mood: In English, we seldom make a distinction when verbs are in the subjunctive mood. But the subjunctive can't be avoided in Spanish if you wish to do more than state simple facts and ask simple questions.
8. Ignoring proper pronunciation: Spanish pronunciation isn't all that difficult to learn, and you should make an effort to imitate native speakers whenever possible. The most common mistakes of beginners include making the l of fútbol sound like the "ll" in "football," making the b and v sound different from each other (the sounds are identical in Spanish), and failing to trill the r.
9. Assuming that the textbook (or this site) is always correct: Even educated people don't always talk according to the rules. Although Spanish according to the rules will almost always be understood, it can lack the texture and sincerity of Spanish as it really is spoken. Once you feel comfortable using the language, feel free to imitate the Spanish you hear in real life.
10. Being afraid to make mistakes: Mistakes are inevitable with learning, and the worst mistake you could make would be to be fearful of using what you know. Remember that no matter how many mistakes you make, wherever you go in the Spanish-speaking world your sincere attempts to learn the language will almost always be appreciated.