This is only an answer for me. A bike that won't do for me, might work just great for someone else. For me, though, I don't think just any bike will do. I'm not riding anything fancy right now, just a basic mass produced, aluminum hybrid (a 2003 Giant Cypress DX). I've always used it for weekend recreational rides, almost always on greenways. Now that I'm commuting on it I've noticed some things that weren't obvious on the nicely paved, car-free paths. First, the ride is kind of harsh. I feel absolutely every bump. Second, the derailleur gearing is much more difficult for me to manage now that I have to stop frequently (why do my stops always seem to happen at the bottom of a hill?). Third, my bike is on the small side, which meant that it could only fit a seat post rack. That limits how much I can carry and means that I can't use panniers.

Yes, this bike is working for me. It gets me to work and back. And more, it makes me happy. So, I guess it "will do." But I also think that riding it has established my low threshold for what will do. A department store bike won't do (for me). A bike with narrower tires won't do (for me). A bike without fenders won't do (for me). A bike that is less upright than this one won't do (for me).

I've promised myself that I can have a new bike if I commute regularly for a year. That bike is going to have a step-through frame so that I can wear skirts and dresses without having to change at work. That bikes is going to have a rack that can take panniers. That bike will have hub-based gears, lights, and brakes. That bike is taking on a very nice shape in my imagination!