I think many of you guys are missing the real issue. It's not so much that you as an individual customer will have to pay more to get higher priority. It's the major carriers providing preferential service to their partners and their own products. For example, Time Warner gives high priority to its streaming video service while degrading competitors like Block Buster, iTunes, and YouTube. Microsoft might strike a deal with Time Warner, guaranteeing priority service for its traffic and degraded service for competitors like Google or iTunes. Maybe Time Warner caves to pressure from evangelicals pressuring them to block porn sites, or from corporate or political interests who want to shut down access to unflattering blogs.
You as an individual customer won't be able to pay more to restore that service because you're not really in the picture. You may not even be aware of the deal. All you'll know is MSN services work great, while Google "seems really unstable these days," iTunes is just "so slow, it's not worth the hassle," and HawtTeenLoveSlaves.com seems to have disappeared entirely.
You might counter, "I'll just switch to another ISP." You're right, that might work ... as long as that ISP doesn't buy its bandwidth from Time Warner, or from a major carrier like Sprint or AT&T who has also signed with Microsoft.
That's the real threat of losing net neutrality. It allows major corporate interests to effectively control your access to content, moving the Internet from its current state of almost infinite diversity into the same bland, commercial cesspool as television. The Internet thrives, warts and all, in large part because it has been a free-for-all equally open to all.