I don’t know about you, but many times when a supposed flash of brilliance hits me, I believe I have the mental equivalent of gold. I tend to get a bit ahead of myself, and start thinking about all of the money that I am going to make, what the product is going to be, how much customers are going to love it, how I am going to sell it, and who is going to buy it. Somewhere in the middle of that process, I start designing the product; making decisions about the absolutes of what the product will and won’t be.
You will find that your first idea is seldom your best one. This is true along two separate axes. First, if you spend any incremental time on design and storyboarding, you are likely to improve upon the original idea in immeasurable ways. Second, if you start letting your mind wander, you may come up with a completely different product/solution that addresses the same need, either directly or tangentially, and does so in a far better way than your first idea.
Just as the notion of testing your idea with only one person is a guaranteed recipe for fail, using yourself as the sole testing point will likely land you in a situation where you could potentially create something that no one will want. This is especially true if you just start building the first thing is that pops into your head.