FWIW, Will, I've not hung out at DI or DU. I've looked in on DU occasionally, but that's pretty much it. So what I "know" of folks over there is from re-posts I see here or CU (generally the more extreme DU-members, I think).
Frank and David Allen (founder of DU, absentee landlord of DI) go way back. To the distant past when we all knew we needed each other, before the politics of hate and fear and personal destruction took hold on the national imagination.
While Democratic Underground (DU) dates from 2001, and its mandate as a networking and news aggregation site for persons in accord with U.S. Democratic Party principles, with a single stated goal, "to elect more Democrats" remains the same (a remarkable achievement for such an early message board to have survived 17 years, all warts and quibbles aside), Discussionist (DI) was started by David "Skinner/Naveed" only 4 years ago, both as a sáfety valve for DU, and as an open site for "freewheeling discussion" and "civil discourse" between all comers.
While DU has faced severe challenges (beyond our present scope; will post O.P. in future), DI faced obstacles David appears to have found insurmountable, and being unable to pay full-time staff to police the joint, has turned his back on the ingrates who turned his well-meaning gift to whatever.
I joined in good faith 13 months ago, knowing nothing of this, and jumped in the water to sink or swim. In so doing, I met some fine people, and learned a few things, which in due course led me here.