Like anything else in business, a business plan should be judged good or bad not in a vacuum but in its business context with its specific business objective. Most of the online discussion about business plans is focused on business plans related to seeking investment, and I`m going to make the assumption in this answer that you are asking about those.
Defining the problem you are solving for your customers is by far the most critical element of your business plan and crucial for your business success. If you can`t pinpoint a problem that your potential customers have, then you might not have a viable business concept.
The best use of business plans starts with founders using plans to establish strategy, tactics, milestones, and (especially important) essential projections of sales, spending, headcount, startup costs, capital needs; it`s for the founders to know, first, what they plan to do.
The executive summary of your business plan introduces your company, explains what you do, and lays out what you`re looking for from your readers. Structurally, it is the first chapter of your business plan. And while it`s the first thing that people will read, I generally advise that you write it last.
Do not misunderstand, these are important inclusions in your plan; however, during the early stages of drafting, it is important to create a broad vision that can be adapted once your specifics have been identified.
Ideally, the executive summary can act as a stand-alone document that covers the highlights of your detailed plan. In fact, it`s very common for investors to ask for only the executive summary when they are evaluating your business. If they like what they see in the executive summary, they`ll often follow up with a request for a complete plan, a pitch presentation, and more in-depth financials.