I've seen people complain when served a beer without foam. Equally, getting a glass full of foam is no use.
When serving a beer, how much is the right amount of foam and why is this important?
Most importantly, a good head helps release the aromas of the beer, especially the hops. Aroma is everything for enjoyment of a good brew. When enjoying a super-hoppy IPA, you should always use a glass that provides a large surface area for aromatics to rise from.
It can also provide the a pleasant mouth feel. Stouts definitely benefit from a thick, silky head.
And lastly, it's eye candy.
As for the right amount of head, it varies. Generally in an average pint glass you want 1-2 finger-widths of head. Some hefeweizens and wit beers are best with a bit more.
Apart from the simple explanation of people liking the foam, the lack of head could indicate problems with the beer. For example:
It could mean that the glass is dirty, or there is left over soap residue on the glass. This can affect the taste of the beer.
It could indicate that the beer has lost its carbonation (the head being formed by the gas quickly coming out of solution in the beer). For many forms of storage a lack of internal pressure will indicate that they are no longer sealed properly, which makes it easier for infections to enter the beer or for it to oxidise.
As for the ideal amount of head, to some extent it will depend on the style of beer.
One of the most important tasks of the foam is to protect the beer against oxidation. This is why some beer glasses have a rough patch at the bottom so the bubbles will have place to "sprout" from. In a Duvel glass for instance it is a laser engraved Gothic "D". When the beer is in the glass (Duvel is highly carbonated) you can see the bubbles rising from the rough spot and creating a protective blanket on the beer, thus preserving the taste long after you poured it.
Here in the Netherlands, we mostly drink poured beer with like 1/4 of the glass filled with foam and we think the beer is not as good anymore when there is no foam in it as the carbonation is gone. But I don't know how it is in other countries, so I believe it is beer and country-specific how much foam there is in a glass.
I must say that before starting to brew myself I disliked the head, but when you start to deal with low carbonated beer, it becomes interesting. Just bought this weird "head maker" an ultra-beer thing it is really weird, looks like a sex toy, but creates head with ultrasound. Just stick it in the beer and press the bottom. With some beers it definitely adds flavor, especially hoppy beers.