Maybe the most natural thing for agree/disagree is a green check mark and a red cross? Then karma could maybe stay +/-.
ETA: I see Emile's example incorporates a check mark and cross also, though there the cross looks a little like it's being used as a check mark.
Design sketches
1. Notes on differences between these images and what we're planning on implementing:
2. Meetup widget in sidebar, listing the 5 nearest meetups occurring in the next 2 weeks, linking to:
(something like right-click & "Open Image in new tab" will help here)
3. New headers: