HN can't become like FarmVille. The key element to FarmVille (and games like WoW) is -- you level up not due to skill, but by how long you play the game. In other words, a very skilled player will level up just as quickly as an unskilled one.
On HN, karma is earned much more rapidly by "skilled" users, even though to get a high karma you must use the site often. Taking this away would rob HN of its most valuable asset -- karma encourages "skilled" users to post insightful comments.
* give you karma just for stopping by each day
* give you karma for upvoting other's posts
* give you profiles you could customize by trading in karma
* show you what your friends were upvoting and their Karma
* show you how close you were to attaining moderator status
* allow you to give one virtual gift per day to your favorite contributors
* have your karma decline if you don't stop by in a week