Low power: it should not require much in the way of network traffic or heartbeats so that you could leave the radio mostly off.
Mesh networking: ideally the devices should be able to repeat commands so that they could be low-power and not have to broadcast across entire houses.
Secure: it should have specific ways to join a network that don't require a keyboard on the device.
IoT application focused: the protocol should make it easy to do things like turn a device on/off, set a value or read a value.
Z-Wave and Zigbee do these things (for the most part), while WiFi is sort of hacked to try to maybe accomplish it. For instance, my Z-wave thermostat has been running off the same lithium AA batteries for about a year now, and the battery level is at 77%. I can't imagine a WiFi device being able to achieve that.