1. Problems too trivial to expend energy and time solving once and for all. (It's easier to pay the small cost each time you encounter the problem). Not marketable.
2. Problems you can solve, but everyone else already have partial or 'good enough' solutions for. Competitive, no innovation.
3. Problems you think you can solve, but you really can't. Doomed.
4. Problems you know you can't solve on your own, or with the resources available to you. Expensive.
5. Problems outside of your area of expertise.
6. Problems you have, but really aren't problems to anyone else.
The workflow is basically:
1. Find an audience ("wordpress consultants")
2. Stalk their forums & chat rooms. Listen to complaints and problems.
3. "Flip their pain" - figure out how you could trivially fix their problem. Nothing more. Don't jump straight to "web app" or even "softwarE" at this point. People in my class were working on shipping well built excel sheets, or books, or trainings.
4. Build that thing in #3, and sell. Normal email / blog marketing techniques apply here. Get paying customers right away.