There actually *are* some interstate highways (up in sparsely populated states like Montana/North Dakota) that don't have set speed limits (done after they repealed the nationwide 55mph restriction for interstates). However, you can still get flagged for reckless driving if you're going extremely fast, or weaving through traffic, etc.
To a point, I don't think driving faster (in light/no traffic, with good visibility and a dry road surface) is more dangerous. The major highways around here (MA) have 65mph speed limits, and traffic regularly moves at 75-85 in the left lane when traffic is light. As long as drivers don't do stupid things like cutting people off or changing lanes without signals, and maintain adequate distance between cars, you have enough reaction time to stop or slow down if something happens. Now, there are stupid drivers that don't do those things, or drive at 100+mph, or who drive too fast in poor conditions or heavy traffic, and they tend to cause a lot of accidents. It might help if getting a driver's license required more than proving you can drive on nearly empty city streets for five minutes.
Frankly, what I think they should do is to stagger the speed limits -- so the left lane has a more realistic speed limit of, say, 85-90mph, the middle lane is 75mph, and the right lane is 65mph (some German autobahns do things like this). Then you could actually enforce the posted limits, instead of only pulling people over when they go 20+mph over the limit.
The idea of having 'graded' licenses or something like that is interesting, but enforcement would be difficult. Maybe if you built something like HOV lanes with a very high speed limit, but you had to get a special license/plate/transponder to use them (and you could enforce it with automated camera systems, like they do for people who go through EasyPass/Fastlane toll booths without a transponder). Seems kind of impractical in most places, though.