The Reddit karma algorithm is already a black box, so there's not much harm in making it more complex, thus:
Measure the general up-votiness of each sub, and scale cross-sub karma based on this value. A +100 post on a sub with 1000 members on a technical topic where people tend to downvote for minor errors is worth way more than a +1000 post on a fast-moving sub that's mostly reposted memes.
Measure the general up-votiness of each sub, and scale cross-sub karma based on this value. A +100 post on a sub with 1000 members on a technical topic where people tend to downvote for minor errors is worth way more than a +1000 post on a fast-moving sub that's mostly reposted memes.