Does best allow you to add prior information?

Supposedly you can add it but you'd have to edit the source, and that's beyond me right now.

You might try adding a prior over the effect size, it would be surprising if it was huge. For example, -30 seems implausibly large to me.

Sure, but the normal distribution is the wrong distribution to be using in the first place. I'm not really sure what... an exponential, maybe?

You could also add priors for the group means. You have some pretty good prior information here since there are lots of other posts. It would be interesting to look at the distribution of post karma.

You'd need the post karma in the first place. Offhand, I don't know any way to get it other than scraping thousands of pages...

perhaps it would be better to do the analysis on something like a log scale? Obviously it can't be exactly that since there are negative values...

Run the log on the absolute value and negate.