Thanks to all who contributed to this thread, I think it has been a most useful discussion. I certainly gained quite a bit out of it. I promised to summarise the lessons at the end, so here goes:

A) Do we judge singer/songwriters more harshly?
Overall, it was felt that the criticism levelled at songwriters is fair, just and constructive. Though it may be a little coloured by the fact that as we become "old farts" ourselves, we tend to favour familiarity and depth in musical experience over breadth. One must also remember that for singer/songwriters there is a "double whammy" of being judged in both roles. And quite rightly too.
A very important point to be made is that in the folk club world particularly the audience is looking for participation - new original songs go against the grain therefore (until they are "sung-in").
But the most important reason why the "apparent" increased criticism of songwriters is in fact fair, is that original material is not yet sifted through the filters of audiences and time (while the other material sung by songers is in fact distilled over the years). So there is a higher degree of mediocrity in original material - and that mediocrity is reflected in the criticism.

B) OK, so what advice would we give to songwriters, to help them create material that would be more acceptable, and to reduce the "mediocrity factor" just referred to above?

For the Lyrics:
Consider FOR WHOM ARE YOU WRITING the song. TELL A STORY, and preferably one THAT YOUR AUDIENCE CAN RELATE TO. Avoid personal stories, LOOK OUTSIDE YOURSELF. BE "Anon" - BE A CHRONICLER. Remember that YOU CAN MAKE SERIOUS POINTS USING HUMOUR. And above all, DON'T WASTE WORDS, SAY THE MOST IN THE LEAST.

For the Music:
CATCHY TUNES WILL ALWAYS BE MORE ACCEPTABLE THAN TRICKY ONES. FIND A "HOOK", something that the audience will remember and could WHISTLE later. And remember - MUSIC SERVES THE LYRICS, not the other way round.

For the Performance:
KNOW THE SONG, and DON'T APOLOGISE FOR MISTAKES. SHOW ENTHUSIASM for the song. USE HUMOUR IN THE INTRO. DON'T OUTSTAY YOUR WELCOME. SANDWICH BRAND NEW SONGS BETWEEN FAMILIAR ONES, DON'T TIRE YOUR AUDIENCE. STRIVE TO IMPROVE AS A PERFORMER.

For the Attitude:
KNOW THYSELF, avoid self-delusion. BE PATIENT and LISTEN TO CRITICISM; use the comments to improve your material, DON'T BE PRECIOUS about your song, everybody has an interest in its being as good as possible. JUDGE AUDIENCE REACTION and abide by it. And above all DON'T WHINE!

Here endeth the lesson. Thanks again to everyone.