Limit the number of links submitted per account per day to 1.
Why
Prevents spammers and karmafarmers from submitting the entire TechCrunch\Wired back-catalog at a rate of 25+ a day.
Further Analysis
Increasing the scarcity of a resource (link submission ability) will increase the value of items it is traded for (links). HNers value independent news related to code or unique analysis. HN already gets the independent submissions people want. They just die an early death on the new page due to overcrowding by links from webzines\newspapers with a profit incentive for maximum linkbaitery. This feature reduces the rate of dropoff for independent news.