From Lombardo, C. Todd,McCarthy, Bruce,Ryan, Evan,Connors, Michael. Product Roadmaps Relaunched (pp. 156-159):

1. Your, or someone else’s, gut
2. Analyst opinions
3. Popularity
   1. "We truly believe that outsourcing your product strategy to your customers is a mistake in nearly all situations."
4. Sales requests
5. Support requests
6. Competitive me-too features
   1. "The quickest way we’ve found to reduce the value of your product is to get into a tit-for-tat feature war with your competition."