I can't seem to turn off D2. In the old layout I much preferred D1, and gave D2 a brief try with the new layout to see if it had improved, but decided to switch back to D1. I performed the following steps:
1. Open "Account"
2. Go to "Discussions"
3. Click the "Classic Discussion System (D1)" radio button
4. See the options change from D2 options to D1 options
5. Click "Save"
and nothing happens. Discussions still show up in D2, and the next time I open up thr account preferences, D2 is still checked as though I'd never done anything. I've tried logging out and logging back in, quitting and restarting the browser ... nothing. This is happening on Safari 5.0.3, as well as an older version of Opera (9.60), on OS X.