Table design question

I'm reading Joe Celko's book SQL Programming Style for the second time
and although I've been an OO developer for quite a few years I'm
fairly green wrt SQL. Joe is obviously something of a curmudgeon and I
would fall squarely into his newbie OO developer ordinal scale and I'm
trying to avoid the slide into stupid newbie OO developer.

So I'm designing a table and I'm looking for an appropriate key. The
natural key is a string from a few characters up to a maximum of
perhaps 100. Joe gets quite fierce about avoiding the use of a serial
id column as a key. The string is unique in the table and fits the
criteria for a key. So should I follow Joe's advice and use my natural
key as the primary key? It sounds reasonable but it will mean at least
one other table will have the string as a foreign key. My postgres
intro book has id columns all over the place but is it really that big
an issue these days to have a 100 character primary key? Are there
postgres-specific implications for either approach?

Thanks
Dave