"Bias" is an interesting factor in design, and also in user / usability testing. I suppose there is a ton one could say about it. But, in general, it's useful to look for signs of particular types of bias, and consider whether there is some bias behind an idea / opinion that needs to, in itself, be explored for the sake of realizing a successful design / project, etc.

Via Rebecca's Pocket, here's Wikipedia's List of cognitive biases. This might come in handy :-)