nothing would really prevent an individual from registering several accounts using different IP addresses and maintain a certain level of activity on each, then

nothing would really prevent an individual from registering several accounts using different IP addresses and maintain a certain level of activity on each, then use 3-4 of them to propel his/her stories to the front page. Not particularly effective. Upvotes are weighted dependent on your karma (and probably a few other factors). The same IP's voting for the same users submissions also flags the detection system. It's insanely easy to fall fowl of the voting ring detector. I've inadvertently fallen foul of it before when the same few work colleagues in one of our other offices were upvoting my submissions each time without my knowledge. As far as I can tell it resulted in my posts dying and their upvotes not even registering.