This is a great comment. While part of working for a start-up is being willing to do work that is "below" you (e.g., doing operations and release engineering from time to time, because it has to be done even though you're a software engineer), at one point you have to ask yourself: suppose this company sinks or has a layoff, will I still feel that I've learned and moved my career forward in the mean time? If the answer is no, politely bring it up with the management first (perhaps they don't realize you're underused and not learning), and if no concrete steps are taken to improve the situation, find another position.
The reason to bring it up before looking (or at least before submitting your resignation and accepting a job offer from elsewhere) is not to be tempted to stay when given a counter offer. If there's a genuine desire to retain you long-term, they should have responded when you brought the issue up; otherwise, they just have a business-critical project they want you to finish and can then let you go.
I do have one comment to make: when selecting a job, don't select by thinking "which company will be the most stable and profitable?". Think, instead, "if all of these companies were to tank in a year, my stock becoming worthless, in which one would I have learned the most?". The "paradox" here is that the interesting working and continuous learning will make one feel engaged and fulfilled, leading them to stay longer and contribute more to the company, becoming less likely to be laid off and earning respect from coworkers and business partners, leading to employment opportunities in case of failure.
Compensation and the perception of a company (especially a start-up) as possessing a unique business idea can also cloud people's brains. A lesson I've learned is that if I can't decide between several offers, as they all seem the "same" (in terms of quality of the engineering team, technology used, culture, interesting-ness of their focus), I should pick the one that seems the least financially lucrative to be sure I'm not being "blinded" by the money.
Paul Bucheit's experience at Google (taking a paycut to join Google, despite being convinced they will fail) illustrates this. He didn't go in thinking "I will build my career and become rich here", he went in thinking "I will learn tons here".