Not displaying your management expertise.
The value of your people will give credibility to your ideas and even to your fiscal projections. If your management team isn't as powerful as it could be, collaborate with an extraordinary board of advisers.

Not demonstrating the cash flow.
What do customers pay you and why do they pay you? Don’t be too cocky in determining profit projections or you'll countermine your credibility.

Not describing your long term costs.
Will it make an actual profit? Will it make money for your company? What is the margin structure? What are the costs?

Not being distinct enough in your product description.
You must let the reader fully understand the demand and the niche for your product. It might be obvious to you, but not so to the reader that’s not informed in your industry.

Not demonstrating the present and future market opportunities.
How big is your market now and what will it look like in 5-10 years?

Not recognizing your competition.
Investors know that if there is no competition, there may be no market for whatever you’re offering. The more you can describe about your competition, the better you’ll understand your market, and the more likely you will conquer it.

Not writing for the target audience.
While the audience is the same, the plan should be described from the viewpoint of banks, investors, and many others. You should even customize each plan to the reader’s particular interest to demonstrate your expertise and know to whom you are talking.

Starting with a boring, passionless executive summary.
This is the first part to be read, and if it isn’t rousing the rest may never be seen. Make it entertaining and be passionate. It should stand alone and yield interest for more. It deserves all the thinking you'd put into a professionally done promotional piece for your clients.

Demonstrating a mediocre presentation.
If the reader notices that you have typos and grammatical errors in your business plan, s/he will assume the business you do is equally the same.

You're saying more than you should.
Keep the plan to a max of 25-30 pages, with an executive summary of 2-3 pages or less. If investors are curious, they'll ask for any other info they need. Amateurs talk about insignificant points because they don’t recognize what they should say and what they shouldn’t. Hire a professional editor to bring down the page count and assist you to accentuate your strengths.

The general who wins a battle makes many calculations in his temple where the battle is fought. The general who loses a battle makes but few calculations beforehand. Thus do many calculations lead to victory, and few calculations to defeat: how much more no calculation at all! It is by attention to this point that I can foresee who is likely to win or lose.- Sun Tzu