The dumbest mistake is viewing design as something you do at the end of the process to 'tidy up' the mess, as opposed to understanding it's a 'day one' issue and part of everything.
Design is so critical it should be on the agenda of every meeting in every single department.
Think about the market, think about the design, and think about who is going to design for that market. Hit the mark.
The market may never coalesce around one basic design, or even around two or three dominant devices.
