[joosters]

If this was introduced, lots of random sub reddits would spring up where bots would post rubbish and upvote each other until they got the required karma.

[pas]

It could be a per subreddit count. Subreddit mods could trust users coming from other subreddits, and so on.

[Angostura]

At worst, we'd be no worse off than now. At best this behaviour would be detectable by admins