Fewer newbie questions generating traffic at PerlMonks and therefore less interest in the site

If the new site can help generate new people using Perl, then it'll end up directing more intermediate/advanced questions.

Mentors with less experience providing less understanding of poorly asked questions and poorer answers

I'll admit that is a concern. But the way people code changes a lot as they get better. Some things it's important to get right form the start, such as form 2 email security. Whereas sometimes the *best* way is to complicated and off putting for a newbie who just wants it to *work*.

Confusion over where to post questions and double posting

Personally I don't see this as much of a problem. A difficult post to the newbie site would just have a response that they'd be better off at PerlMonks.org

Less incentive to improve PerlMonks interface

I don't agree with this. If the newbie site looked better, or functioned better in any way then I'd think that's much more incentive to improve the PerlMonks interface.