Signal Strength. I have a wired connection to my bedroom that terminates in an Airport Express. Helps when streaming video and for games having the access point within 8 feet of the router. Within that 8 feet is 9 of my own devices (2 laptops, iPad, iPhone, Vita, PS3, Raspberry Pi and Apple TV.. sometimes 2 Apple TV's).
Then add on sometimes my Fiancé's iPhone and Laptop. That's 11 WiFi devices within my tiny 10ftx12ft bedroom.
That's why I've cabled my place, wireless really suffers with multiple devices unless you spend big bucks on a suitable access point.
I've sometimes had to draw pictures for business clients who believe a G access point will provide a decent connection for multiple users, some people have this belief that if it claims to provide 54Meg speeds then that's what all users should get, and don't forget the 90 metre range, and lets ignore architectural features like walls, concrete, roofs, etc, etc, etc......