If you need to ask...
Well practically on a modern system especially as a server of some sort, you can get by without any at all - You can go with onboard video (avoiding the use of a PCI-E slot for video card), ethernet (so you don't need a expansion slot for it either) and so on. Most modern boards are practically self contained.
As far as a home server system goes, I can see PCI-E slots used for a card that supports additional sata ports (assuming you had space and sufficient power in your case). I'd note that many MATX cases might not support the full 6 drives that the board could handle - most i've seen handle 2+2 or 3+2.
If cost was no object you can throw in a SSD or even ram on a PCI-E port. Both these act as expensive but INSANELY FAST storage options. Considering you're using a matx board, i'm assuming throwing a grand into very fast storage is not an option you're looking at, so lets ignore these.
If your server also does some form of routing which is common, you could also throw an additional network card (PCI-E for gig-e, PCI for 10/100). I often use ethernet as a simple fast way to transfer files directly from system to system as well. Having wireless is another possibility in that role.
With an MATX board you can't expect too many cards - you'd have one x16 (for a video card) and up to 3 other slots as per the specifications. On the other hand, in this role, nearly everything you need is built in, as i stated - I'd look at other specifications in deciding.