I was thinking of something like (upvotes+1) / (downvotes+1). Adding 1 to denominator is necessary to avoid division by 0. Adding it to numerator too balances it out and makes varying number of downvotes visible in spite of 0 upvotes.

Controversial posts would get a score near 1 instead of fluctuating. You would also get a rough idea of intensity as more ratios become possible when the number of votes increases.

I think it's still simple enough to be transparent to most users.