I have two experiences here that may be relevant.
Some years ago I was in a comparable position, only I was working full-time at a salaried job when I was approached to be a partner in a new startup. The elevator pitch and cocktail-napkin numbers were engaging, and it looked like we could bootstrap it with resources we already had on hand and be profitable within a year.
Except that six months in, we had no business plan. Now, I am not an MBA type, and I think that the value of the business plan is more that it's a self-check: if you say "Based on our projections, we should have 100,000 users and be realizing $10,000 a month in subscription fees and ad revenues by December 2012," and in December you have 35 users you know something is wrong; and if in December you see $1.2 million in revenues then you know you're doing something right and you'd better start doing more of it quickly.
Without a business plan on paper, we had no way of gauging even that the founders were on the same page as to what "success" meant, let alone whether we were succeeding or not. After six months of asking for this, I said, "I can't work a second full-time job on spec, and my business questions are not being taken seriously because I'm just the guy who knows web servers and streaming video." And I walked.
You might try an approach like that: pick up a basic business textbook and ask questions. When do they expect to see revenues? Do they have an exit strategy where they make themselves attractive to a large company and get bought for kerjillions of dollars? Do they have a non-exit strategy where they bootstrap themselves to profitability? If they can't answer that question, get out before the paychecks start bouncing.
My other experience was with a startup. It was five non-technical people - three who had lots of experience in the problem domain, one who had experience in business development, and one who had experience in data processing software development for insurance companies and bill collectors. And the goal of the startup was to put a certain experience online. The three experts in the problem domain were full of (and I say this without irony) excellent ideas as to what the software should do.
The principal structural problem that we ran into over and over again is that the only partner (and thus the only voice that was taken seriously) who had any experience in software development came from a domain where the business analysts would issue a mostly-clear set of rules and it was the job of the programmers to turn that into COBOL. So their first approach was to hire a consulting company to build the product for them. That was an almost completely unmitigated disaster: the only real benefit was that it let them get a product off the ground and start realizing some revenue. This was offset by the fast realization that the consulting company was milking them for all they were worth because the consulting company didn't expect them to last, which then led to a very acrimonious parting of the ways followed by lawsuits. And then they hired their own developers.
So the pattern we fell into, which you will probably find strangely familiar, was that the five partners would gather in the conference room once a month and come out with with two or three good solid ideas. They would then instruct the developers (two coders, a web designer, and a database analyst) to make it happen. We would start on it, hampered by the horrendous infrastructure that the consultants had left (but there was no time to address technical debt). We would get maybe halfway through implementing one of the ideas, while putting out fires and serving as tier 3 tech support, and then the next monthly meeting would happen.
That was the job where I learned to give development estimates in person-days on task, rather than in calendar days, and where I picked up the habit of automatically appending "assuming you don't change the requirements in the interim, because that will make it take longer" to estimates.
So what happened there? Well, I was the first developer to leave. Within a year, all of the developers had left, the partner in charge of software development had also left (no doubt with a massive golden handshake for her efforts) and the company had been folded into one of the CEO-partner's other companies, possibly with the recognition that it would never be fully profitable.
Based on my experience: if there is no technical person competent in the development style you're working in among the partners, if the only technical people are employees rather than partners, stay away. There may be exceptions, but generally partners have a lot more credibility with other partners than employees do, and if you as an employee are saying "this will take X person-days to implement and will not be profitable" and a partner is saying "this will be trivial to implement and I read in Infoworld about a company that did this and made ten billion dollars a day!" then the partner will be believed and you will not. No matter how many times you are right and that partner (and Infoworld) are wrong.
Summary of the summary: no technical partner means you shouldn't have signed on in the first place; no path to profitability and no exit strategy means you should get out while you can.