I came across this quote from Michael Dell in an old Harvard Business Review (3/05, “Execution without Excuses”):

“When someone runs in to our offices saying ‘We’ve got to do tablet computers,’ because Microsoft says we should, or because everyone else is doing them, we ask, ‘How’s this going to work? How big’s the market? What’s it going to cost us? And do we have the operational bandwidth to handle it?’ Because you can’t do everything at once and expect to succeed across the board. We are not looking for the most challenging problems, we’re looking for the easiest problems that have the most opportunity.“