TL;DR ahead
First, what we know:
1. Two eyes, at least human-ish. Possibly featureless, but with the mask, it's hard to know for sure.
2. Humanesque nose. Along with the eyes, this is very clear through the mask.
3. Quit it with the damn cybernetics. Tali makes it clear on the elevators that Quarians are not cyborgs.
4. Ears are different.
5. Quarians have similar mouths to humans. How do I know? Their voices. The only distortion on their voices comes from the echo caused by their masks. Otherwise, they sound pretty human. Only the Asari and Humans don't have weird effects added to their voices. Turians, Batarians, Salarians, Krogan, they all have different looking mouths, and they all have added voice effects.
Now for speculation:
1. Hair is a human thing. To avoid making aliens just be humans with bits glued to their faces and funny bending legs, humans needed something unique attached to their heads. We got hair.
2. Regarding skin tone: They will be a pale color, because of the constant lack of exposure. Judging by the general visible tone through the mask, it is probably a cool color, like blue, purple, green, or gray, rather than red or orange. Alternatively, skin tone may be telegraphed by the visor color. Not all Quarians have the same colored helmets.
3. No room for tentacles or anything like that stuck to them. The head is the only part that doesn't appear to be painted onto Tali, and even that is small. The back of the head doesn't have much room either, as you can see some Quarians don't wear hoods, and the back connects to various tubes for the suit functions.
4. Quarians are generally slim. I imagine this continues on to their faces.
Tali herself is apparently attractive enough to warrent a look on Shepard's face that just says "I win" when he removes her mask.
TL;DR:
Quarian faces are skinny, pale, hairless, earless, but otherwise human looking. Also, they're not damn cyborgs (no matter how awesome that would be.)