I take issue with this sentence. fun is very much a relative concept. Some people find exceedingly deep, historical TBS games fun, many find blasting aliens in Halo the ultimate fun experience, others find sneaking around without killing anyone to steal their stuff fun, a few enjoy managing virtual democracies in game form, others still find talking freakish, big eyed, big breasted ladies with multicoloured hair into undressing themselves fun. Fun is in the brain of the enjoyer, to make a horrible twist on an old expression.
What every dev team has to find is a way to balance the game they want to make, with the game their target audience wants to play. As such, it's really a question of what kind of audience you're targeting, and how large do you need that audience to be so that the game you are developing can pull a profit.