One of basic tenets of the Internet has been that it is merely a network for the transmission of packets of data. E-mail, the World Wide Web (ie web pages), online chat and file transfer are just examples of applications available that use the Internet. In other words, the Internet inter-connects disparate networks, such as your work network and my home network, to each other. It is a transport infrastructure, maintained by governments, QUANGOs and private organisations, in a manner very similar to the telephone system the world over.
Does the Internet care if you are reading a web page, watching a video from You Tube (http://youtube.com), talking to a friend over Voice Over Internet Protocol (VoIP) or playing an online game? Normally it doesn't. The base Internet Protocol ("IP") carries the data, which might be TCP or UDP, and your applications determine the rest.
A lot of Internet Service Providers (ISP) don't own any of the infrastructure - they resell what they purchase to consumers. If you deal with a small ISP you are probably simply using a larger commercial provider such as BigPond. Other providers such as Internode, TPG and Exetel own some parts of the infrastructure. But let's say your ISP also is a phone company. If you start making lots of VoIP calls that don't go through the regular phone system then that revenue starts to dry up. I have reduced my Telstra phone bill by 2/3 since I started to use VoIP. So, what if your ISP-Phone-company starts treating your VoIP data differently to web pages? Maybe they'll slow down your VoIP data, arguing that it is slowing down other services such as web pages and email. Oh, so your VoIP calls are poor quality now? Sorry, you'll have to go back to using the ordinary phone line... I don't believe this is happening in Australia at the moment, but it's technically possible. In 2004 in the USA Madison River Commnications blocked their DSL customers from using the Vonage VoIP service. The FCC fined them and things were fixed up, but network neutrality is still not law in the US.
Another example is that of Peer to Peer file sharing networks, such as Bittorrent (http://en.wikipedia.org/wiki/BitTorrent). These allow people to easily download and share files, and since in Australia we generally don't pay for data uploads (only downloads), a lot of people have found this very useful. And no, a lot of the files available through bittorrent are not illegal. However, in an example of this packet/traffic-shaping, the Australian ISP Exetel (http://exetel.com.au) recently decided to slow down by 50% all P2P packets from noon to midnight.
Exetel was a good ISP to me when I was with them, but this seems to be a direct attack on "network neutrality" as it applies to consumers. Not many businesses use P2P for time-critical data, so it's the average Joe who is hit by this. I'd like to see what Exetel's Terms of Service say about their ability to do this.
Perhaps your ISP offers, as does mine, cheap phone calls over VoIP, but you want to use a different VoIP provider. Could they choose to throttle all data to and from that other provider to make it effectively unusable? If Exetel can slow down P2P, why can't it slow down other company's VOIP offerings? There's no suggestion that it is going to, but again it is technically possible.
What's more important?
Let's assume that P2P does slow down, say, VoIP traffic. So? Which one is more important depends on your point of view. You're home bound and you rely on the Internet for your entertainment - P2P is more important to you.
Another example might be that, say, Yahoo could pay more to its service providers for higher speed service end-to-end than other smaller search engine companies. Where's that level playing field? It depends on how much of a free-market economist you are.