IANAL, but I did just look up what looks like the current version of the relevant part of the DMCA[1]. Unless there's one of those funny legal things I don't understand going on, it appears fairly black and white that failing to provide an e-mail address for the designated agent basically nullifies the entire safe harbor provision (see (c)(2)(A)). Whether that is actually what Y Combinator are worried about we don't know, of course; I can't be the only person wondering what prompted this change on a site like HN where DMCA notices/abuse don't seem likely problems under normal circumstances. Or maybe YC's lawyers just know more about the law than I do. :-)

[1] http://www.law.cornell.edu/uscode/text/17/512