Expansion slots are the slots that expansion cards plug into.
Video cards are one of these devices.
For some users the expansion slots on ATX boards are required(SLI + sound card + wireless,ect).
So if you do not need the expansion slots that is not going to be a down side for you.
Just make sure the board supports the cpu you want to use and have a beefy enough power system if you plan to overclock.
For instance, this is my board. I did not need the extra expansion because I just have 1 video card and 1 sound card.
Maximus IV Gene-Z