In a fair and differentiated reading, Keynes's theory has undeniable strengths: it takes disequilibrium, uncertainty and "animal spirits" into account. But Keynes did not foresee that there could and would be a political cycle, with the incumbent government doling out fiscal presents just before elections and cutting back right afterwards. He didn't analyse what the multiplier depends on and therefore didn't imagine that it could be below 1 or even negative. He didn't expect that we would some day find ourselves in a situation where people save little and governments spend a lot — ideal conditions for the multiplier — while that pessimism would stifle the economy precisely for that reason. He didn't take the time lags of political action into account. He ignored the fact that public spending tends to crowd out private investment. 

These are not just minor flaws that he couldn't foresee. These are serious shortcomings that are to be remembered and borne in mind as a caveat against the vulgar Keynesian policy recommendations that now seem to be the staple of governments around the globe. It is hard to believe that Keynes himself would ignore these caveats, were he still alive. While he would certainly have endorsed the immediate reaction of fiscal and monetary policy to the outbreak of the crisis in 2008, it is difficult to believe he would still be in favour of loose money and deficit spending today, five years on. He would urge governments to put on the brake. Crisis policies are for times of crisis. He would know about the bubbles that governments are creating anew. And he would take the risk of inflation seriously, as he always did. Keynes would be wiser than the Keynesians.