Finally a thread discussing something truly important.
I voted for brackets. I think square brackets speak with editorial voice rather than the authorial voice of parentheses, and, as +ifying a thread is taking an editorial license with other people's commenting options, square brackets seems more authentic to the +'s function.
Just throwing a + out to the winds unprotected is, of course, unseemly. Naturally it's only so long until some maverick or renegade uses curly brackets or chevrons, but such innovations should also be condemned as meaningless egotistical affectations.