It's not good to focus on negative aspects, but there are a few really common mistakes new business owners make.
The first mistake we've already discussed, so we won't go into detail, it's not having a plan. There are businesses that have survived and even thrived without so much planning, though those cases are far and few between, also had they made a plan they probably would have thrived quicker than just making decisions on a whim. After you have your plan you should be sure and protect yourself by filing for an LLC or Corporation.
Spending too much money: A big mistake that new business owners make is spending too much money, on decor, furniture, employees, office supplies. Sometimes people think having more is better than not having enough, but when you're on a tight budget, you might not need that extra computer or extra employee. It's always a great idea to start out with things you absolutely need, if you can't handle the business without an extra computer or employee, get another, but it's important to weigh out your supply and demand, there's no need to produce 1000 products a week when there's only a demand for 500 a week.
Marketing: We've discussed this also a bit, but it's important to have a marketing plan, too many business start and no one knows about them besides their family and friends, who will often ask for hand outs. Whatever your marketing plan is, be sure you have one, an effective one that will get the word out to people that you exist. Since we live in the age of the internet, it's easy to do this at lower costs than in the pasts, you just have to know where to put the word out.
Mis-pricing Goods: Another common business mistake is to mis-price goods, pricing them too low or high, it's important to not undersell yourself, also be sure you have a quality product, if not you risk the chance of losing repeat business.
These are all common mistakes, and if you've made one already, you should plan to correct the mistake as soon as possible. Good luck!