Alex Massie has it:
My sense - from his own writing and what I've read about him - is that he is also difficult, stubborn, awkward, cussed and not to be trifled with. these too may not be attributes best-suited to a national campaign in the modern political era.
What he is, however, is something more important: he's clearly his own man and, crucially in this political era, a man one can respect even - or especially - if one disagrees with him (eg, on trade). There is, to use the word that came to dominate his 2006 Senate campaign, an authenticity to Webb that most politicians would chew their right arm off to possess.
P.S. Yes, that's a dated photo--his official mugshot from the Reagan DoD.
Also: See Webb frankly discussing why he thinks the legacy of affirmative action is what's killed Obama in Appalachia.