After searching around a bit and taking what I found into the context of what WiM was saying, here are my thoughts...
It seems that "mainline Protestantism" is a term going back 30, 40, or 50 years that was used to describe some specific Protestant denominations and Roman Catholics who thought of blurring the boundaries between themselves--step across the aisle so to speak--and embrace a kind of multi-traditional ecumenicism. This didn't work, however, because instead of churches becoming more "diverse" in terms of their traditions and boundary markers, they became more "restrictive" and "self-defined." Most Christians didn't want to give up or blurr the things that made them distinct and special, so their reaction was in the opposite direction, to leave those kind of churches and go places that better defined their particularities against others instead of, what seemed to them, giving up their important distinctions. Vatican II was the Catholic response in the opposite direction, pretty much destroying the possibility for ecumencism between a Catholic and a Protestant.
So this would seem to be something that WiM means when he refers to a "moderate" Anabaptist... An Anabaptist who is okay with giving up or blurring some of the boundary markers between themselves as Anabaptists and others in order to engage in a more cross-tradition ecumenicism. And when one looks at the early Anabaptists, one can indeed see that such a thing would be a radical departure. Early Anabaptists saw what distinguished them from other Christian traditions as not insignificant or not something that could be blurred (like maybe it's okay to sometimes use the sword for defense or maybe if you're Lutheran you can slaughter people if they're being really, really naughty), but rather, Anapbatists saw their distinctive qualities as definitive of the reborn, penitent, and obedient Christian who would be saved from the final day of YHWH's judgment and wrath.
I am certainly not "moderate" in that way. I have no desire to become ecumenical. I truly believe Anabaptism has got it right in a definitive way that the rest do not and, like the early Anabaptists, I see those who would differ from us in our distinctives to be headed down the path that leads to oblivion.
I would say that I am moderate in that my Anabaptism, although it takes the early Anabaptists as its basis and foundation, has expanded out from there in ways that early Anabaptism could not (not that it would not). For instance, few early Anabaptists were learned and scholarly. They did not have the time, resources, or safe havens that other Christians enjoyed to pursue an understanding of scripture. But their godless enemies did. The centers of intellectualism did not promote Anabaptism, but produced its sophisticated theological arguments and works against it. Anabaptists therefore naturally associated scholarly knowledge and learning with blindness and corruption. If Biblical truth could be known through learned study, then certainly the learned would not be writing false teaching and supporting the abolishment of what Anabaptists considered divine truth. For the early Anabaptists then, Biblical knowledge was something that came by realism and revelation. All they needed to do to know the truth was to open up the scriptures and read it (realism) and the Holy Spirit would guide them into all truth (revelation). Menno Simon, for instance, writes in many places of the "plain" meaning of scripture. There is, for the early Anabaptists, nothing really to study or figure out because it's all plainly in view.
I have advanced beyond this limited view of scripture not because I am veering away from Anabaptism, but because the faults of early Anabaptistm's original perspective have now themselves become "plain." Scholarship, reason, intellectualism, and learning are not inherently against or blind from truth. Modern science only exists because people believed that a reasonable and intelligent Creator had given them their reason and intellect in order to understand real truth about the world and themselves in it. It just so happened that when the unlearned, early Anabaptists appeared on the scene, learning and intellectualism was turned against them and against divine truth.
The scriptures are written according to the perspectives and worldviews of its authors, which we do not share. To best understand scripture, therefore, means we must understand the ancient context out of which the scriptures came. We cannot simply read them from within to our culture and context and know what they are saying. What is plain to a 1st Century Palestinian Jew cannot be plain to a 16th Century Dutch gentile. Fortunately, the early Anabaptists had the Holy Spirit to help make up for their deficiencies, but that doesn't mean if, given the historical opportunity, the early Anabaptists shouldn't or wouldn't have tried to fix their deficiencies.
Or, again, I would say I'm moderate in that I support the full and equal participation of women in the church body--yes, even as pastors...or because I reject Trinitarianism and the Hypostatic Union. All that is not because I am leaning away from Anabaptism toward something else, but, again, because I have been able to reach a place that early Anabaptism could not (not that it would not).
