Arguing with libertarians about net neutrality is like trying to convince the Pope to open a condom factory. If you succeed, you've converted them. Belief that an unregulated market is always best is a fundamental libertarian plank. With the Pope you actually have a head start because he has admitted that there might be situations in which using a condom might be moral.
I'm not naïve. I'm no fan of state power. But neither of corporate power. The world in which we live today is a complicated one, in which corporate and state power are often intertwined or interdependent, but sometimes antagonistic to each other. A properly crafted net neutrality law would limit corporate power in the critical arena of internet access, and as a consequence would limit state power as well.
A negative law that said "ISPs can't discriminate on the basis of content" could do this in a minimal way. The primary method of enforcement could be individual lawsuits. This would enable one customer to achieve, through the courts, a result that otherwise could only be achieved by massive coordination of many customers. In fact, this would probably rarely require the courts - the threat is sufficient. No doubt civil society watchdogs would do most of the work, leaving the state with little to do.
Let me give a pertinent analogy. In December 2010, Visa, Mastercard, and Paypal all blocked their customers from donating to Wikileaks. Because those three effectively control internet money transfers, we have few other options. Want to buy stuff on the internet? Then good luck boycotting those companies. We cannot achieve the goal of forcing those companies to give us financial access to Wikileaks even with massively coordinated market action.
But if there were a law forbidding a financial company from cutting off an entity without a conviction of illegal behaviour, then Wikileaks or perhaps any customer of Paypal could sue Paypal to force it to allow donations to Wikileaks. Such a law would reduce the power of those three companies. Here's the neat trick, though: even though it does so through a law (state power), it would effectively reduce state power, because the US would not be able to quietly suggest that those companies cold-shoulder Wikileaks.
Similarly for net neutrality. If companies were allowed to block arbitrary websites, they could block our access to the leaked diplomatic cables. The US government could also "suggest" that they do this. If the companies were forbidden by law to do so (as they are now), the US could not do this - and our freedom to communicate would be greater.
The internet has opened up new areas of freedom. It is up to us to defend them in the face of threats from both state and corporate power. Letting capital rule in the form of "pay-for-play" will corrode those freedoms.
Some things should not be left up to market forces. Most people understand this intuitively, and it is our task to convince them that service providers' discriminating by content is one of those things. A small minority of people take it as an ideological, bedrock belief that the market is always best. Argue with them if you want, but realize that it will very rarely be fruitful. If you succeed, it will be akin to a religious conversion.