The insight can't be that people can be unemployed even when people are acting rationally, yet that is the way I read what you wrote.

The insight is that! Microeconomics says that wages demanded and supplied will adjust to reach full employment. Macroeconomics points out that adjusting wages will change consumption and gdp and change demand, and hence company profits, and hence more jobs will be created, destroyed, or repriced.

Full employment is only possible if both these things balance simultaneously. Usual Keynsian approaches posit some friction or irrationality that prevents them balancing (or at least balancing fast enough). But in some cases, it might just be because there is no convergent simultaneous balancing for both processes.

In that case, even if frictionless and rational, unemployment will persist (probably going through wild gyrations) because there's no way of getting to zero unemployment, and it wouldn't be stable even if you reached there. If we're really unlucky, we could get a strange attractor (http://en.wikipedia.org/wiki/Attractor#Strange_attractor).