I agree with Veks, the issue isn't just what they can start charging people more for, it's that the major internet companies have monopolized whatever section of the country they're providing service to. If I think Walmart's car repair shop is crappy and overpriced, I can bring my car to another shop that'll do better work and/or do it for less, meaning Walmart has to fix their prices and service so that people go to them instead of the other shops. With internet, if Comcast is a horrible service, my options are pretty much "suck it up" or "don't have internet", so they can do whatever underhanded practices they want. (Note that while several major cell phone carriers also offer wireless internet service, it's not a very viable solution for a lot of the new internet technologies coming out due to spectrum crunch.)
Further, it's worth noting that most major ISPs (especially the big two, Time Warner and Comcast) are also cable network companies. Their competition isn't just other ISPs (which are next to nonexistent), it's other forms of entertainment, especially video sites - Youtube, Netflix, etc. Giving the ISPs the ability to control how fast data from each site goes is like telling Walmart it can be in control of highway maintenance for a city - expect excellent roads leading to Walmart, and absolutely no maintenance on the roads leading to Target.
Finally, the last thing I'll say: despite what the corporations say they will and won't do without Net Neutrality, the main reason I can see for keeping it is because there's a risk that someone will take advantage of it if it's gone, and I see no good reason to not keep it around.