Successful teams have strong personalities -- when brainstorming, these personalities can become caricatures: the naysayer, the vulcan fact-checker, the polyanna, the emotional chest-pounder, etc. Bias squelches creativity. Good ideas, cloaked in emotion, never see the light.
One way to take bias out of brainstorming is a methodical approach. There are many structured brainstorming processes designed for long, complex product development cycles, but small processes and small companies can get bogged down with all the details.
An approach that's worked for me over the years is the one of "thinking hats" where everyone on the team has to take on different roles (or "hats") to think through the situation from multiple angles. The red hat is the emotional hat, the black hat is the devil's advocate, the white hat is fact-based, the yellow hat is the sunny optimist, the green is the creative hat and the blue hat is the integrator that brings them all together. The method is 40+ years old, but it still feels fresh.
The book is short, once you skip the overly congratulatory introduction by the author. Add a Post-it Note easel pad, place the six points of view around your conference room, and make everyone try each role at least once -- you'll have a more balanced view of your situation -- and a better end product as a result.
Who wants to be labeled "doom & gloom naysayer" or "naive optimist" anyhow?