Minor spoilers for the book follow.
McCarthy is notoriously reticent about giving interviews or offering interpretations on his work. So it's very hard to be certain about his intentions here. But it is noteworthy that both the major characters in No Country For Old Men, Moss and Anton Chigurh, have peculiar names.
In the case of Moss, his first name, Llewelyn, is derived from old Welsh and means "like a Lion". The name was born by two 13th century Welsh nobles, Llywelyn ab Iorwerth and Llywelyn ap Gruffudd, both of whom played a key role in trying to maintain Welsh independence from England. It is, thus, a hero's name, both in its derivation and the history of those who bore it. So we can perhaps deduce that McCarthy wants us to see Moss as the "hero" of this story rather than the other main candidate, Sheriff Bell.
It's also significant that the name is derived from old Welsh. Moss' surname also has connotations with age: moss is an extremely primitive plant that dates back to the earliest colonisation of the land, some 400 million years. We presume forests and buildings that are covered in moss to be ancient.
This would seem to tie in with the title of the book, No Country For Old Men. It's very easy to presume that it's a reference to Bell as the obviously older man among the characters. But it is in fact Moss who suffers most out in the country, not Bell. It may be that Moss' name indicates that the title should be understood as it not being a country for Moss, who is tempted by the drug haul and comes to grief, rather than Bell, who survives and is portrayed as having learned and grown from the experience.
