Attention to detail. A plan that is concise and clear reads professionally so don't make yours too wordy. The reader needs enough detail and information to be able to make an informed decision. As with all professional documents, care must be taken to avoid spelling mistakes and use correct grammar and punctuation. A plan that makes absurd or unrealistic assumptions is of no use so stick to the facts and make sure you are making credible projections and accurate content at all times.
It should include a market research that identifies your competitors, their share of the market and the range of the products they produce. By learning how they conduct their operations, you may learn tricks of the trade in the business you want to enter and you also get to have a basis on what you can do to excel.
A brief account of how the company began- You should mention in your business plan the history behind the formation of you company and the background of the founders and directors.
Finally, a business plan should be error - free. This is important because your business plan defines who you are as a business person. If it turned out sloppy, then that does not speak too highly of you.