there selling points to buyers and gamers. most time they have higher end ethernet chipsets and sound cards...some of the newer mb like msi use extra grounding on there mb to keep headphones and speakers from buzzing. other then that the gaming mb may have higher end caps and vrm (power) to handle the gaming stress without falling.
when you look at mb or read up some have 4 fase vrm..higher end may have 6 or more. the more fases the more stable the power is going into the mb. the last part is gaming mb may have brighter colors or led on the mb. or with asus rog they have heat shields.