cpu socket,space around cpu socket for better airflow,ram slot placement and no. Of ram slots (more is better),chipset model north bridge, pci slot and pci-x slots for sli and cross fire, min 6 usb port, e-sata ports ,diffrent disply connectivity options like vga(old) hdmi,d-sub, dp.
Finally but very important is capsitors build quality.
i hope this'll help u to understand a mobo.