I don't think a +/- system is the way to go because most suggestions get +'s from me, but some are urgent and some would only be nice providing we could have everything we want. Instead, I'd want to see:
Red Plus / Blue Plus / Minus
Any user is only allowed to have five red plusses at any time and no user who doesn't have either a highly rated story or 200 posts can vote at all.
Red Plus: Wanted and prioritized
Blue Plus: Wanted but not prioritized
Minus: Unwanted
A user who is eligible to vote can give 0-5 red plusses, and as many blue plusses/minuses as they want, though obviously, they can only give one symbol to each suggestion.
Then, ideas would be sorted first by red plusses then by (blue plusses - minuses). New ideas temporarily get the top spots or have their own tabs.
When an idea is accepted or rejected, red plusses on it are returned to their owners so that they can spend them on other ideas. Additionally, one can remove a red plus at any time and place it elsewhere.