I have written business plans for all manner of industries: a coin operated jukebox company, airlines, travel companies, new product launches, and anti_aging product companies. It is not necessary to have a passion for the product or the company to write or develop a business plan. What you must have is a passion for aggregating information, getting involved with and understanding the service or product, and understanding the financials of the product or service. By financials I am not referring to having a CPA before you undertake the task, but rather understanding the presentation of the information and analysis/ numbers to support the activity being planned. Financials are important because they are the score card in the world of commerce.
The "One Pressing Issue" Plan: Business planning does not stop the day you open for business. Under the best of circumstances you should be revisiting your plan once or twice a year to see how things are going, and where perhaps you've veered away from your original goals. Remember, changing the direction of a business isn't always bad, but it should be intentional.