Well, stability should be very important. Next features you want at a price you are willing to pay. Personally, an extra RAID controller that can be used as extra IDE ports is a big one for me, and on-board LAN is something that's growing on me.
Firewire and USB 2.0 are relatively new, but pay attention to what chip is being used to power these. While I typically don't mind Via's chips, their USB 2.0 ones are supposed to really suck.
Any P4 motherboard that features ICH4 will have USB 2.0 as it is part of the motherboard chipset, and doesn't require an extra controller chip.
If the USB 2.0 is all you really care about, but still want something good, the MSI 845E Max series of boards would be up your alley. MSI tends to make several variations of the same board, but these ones do feature the ICH4 and an Intel motherboard chipset.
These also use DDR, which is what you want in the first place. You can find them for under $100 online. Personally, I recommend
Newegg
They have pretty good prices---not necessarily rock bottom, but ususally pretty low, and great service.
I don't know how often you plan on upgrading, but it'd be a wise idea to make sure whatever motherboard you buy supports the 533MHz bus P4's. That way, you'll have a better upgrade route at a later time and have one less component to purchase.