The principle of net neutrality from Wiki:
The principle states that if a given user pays for a certain level of internet access, and another user pays for a given level of access, that the two users should be able to connect to each other at that given rate of access.
This would prevent ISPs from filtering or slowing content from some sources, which is censorship either in content or bandwidth form.

In the US, if I'm not mistaken ISPs are generally considered common carriers, which means they are not responsible for the content transmitted on their lines because they are supposed to be ignorant of it. Filtering or access tiers breaks this.

If net neutrality is ditched, essentially the major content providers and ISPs will collude and turn the internet into what TV is today. ISPs will be able to extort money from sites, and the independent wackiness of the internet will disappear.

Quote Originally Posted by Crazed Rabbit
And besides, what ISP would throttle access to certain sites? People would just abandon them.
You are making the assumption that people have choices in their ISPs. Many areas are stuck with either random dialup services, or their cable company.