'Reddit does a pretty good job of defending against blogspam...'
From the comments on 'There's a fine line between blogspam and self-submission'
Reddit does a pretty good job of defending against blogspam due to its Karma system. If you submit too many posts that get voted down, your karma can go negative. A high karma means a submission has a better chance of making it to the front page. A low karma means your submission might not even get seen by many people.