Yes, there are many common mistakes that people make when they are learning to write, so I will only address a few here:
1. Not keeping your characters straight.
Too often new authors forget who their characters are. One character will not only begin talking like another character, but also begin acting like another character I highly recommend creating a character sheet before you begin writing that explores each character in detail (what do they like/ dislike, how do they talk, what is their education, etc.). Also, keep main characters to a minimum. Everyone whats to write an epic fantasy like the Lord of the Rings, but maintaining that many characters is very difficult.
2. Repeating things.
Rereading what you wrote is important. Not only because you can catch a number of typos, but also because you can see where you repeat information. Sometimes, new authors will even repeat things almost word-for-word. The only way to check for this is trying to read everything you have written in one setting (or as close to one setting as you can).
3. Not running a spell check.
This is one that really floors me. Spell check is simple and easy to do. You press a button and voila! Okay, so you do have to make sure the spell check is flagging true typos and not stylistic or non-errors. It isn't as smart as you. Still, it is simple and keeps you from being very embarrassed because you wrote paritcle instead of particle.
4. Forgetting your theme (or not even having one).
What is the theme of your book? This should be something brief like "to enslave" (the theme of my husband's book, the Corruption). This theme should drive the story. In Sal, Captain of the Baby Guards the theme is "to be brave." The whole driving force behind the book is to bring the main character to the point of overcoming his fear of dragons. Some books can have themes as simple as informing people about a topic; others can be more complex.
5. Forgetting your purpose for writing.
Along those lines, you need to keep focused on the reason you are writing. George Lucas seemed to lose some of his purpose for writing in the last series of Star Wars movies he did. He originally wanted to entertain people by creating an epic space fantasy. By the end it seemed he only wrote to make money off of an established fan base. Never write just to make money. If you don't love it, if you aren't passionate about your topic, you won't fool your readers.
6. Forgetting your genre; forgetting your audience.
Similarly, you have a genre and you have an audience. You can create a crossover genre, but if you are trying to write a picture book/ epic fantasy/ erotica, it won't work, and it will be extraordinarily difficult to market. Yes, you can have some crossover, but if you can't classify it according to two or fewer classifications, you should limit yourself. Some people don't like limits, but if you can't write within limits, how do you expect to write well outside of them? I can't think of any teacher who opens her writing class the first day with: "Okay, I want you to write something. You have five minutes. Go." Think about where your writing belongs, and then think about the people who will read it. Your job is to give them something they want to read. Follow the rules and people will want to read what you wrote. Yes, you can get your relatives and the people who like/ love you to read anything, and unless you have my relatives, they will probably tell you it's great. However, you don't want people to read your work because they have to- because you offered it free to a school and they feel obligated to assign it. You want people to read your work because they want to read it.