Anything ES6y implemented in modern versions of V8 that aren't in the version that Node.js is using cannot be used. E.g., if you use `"strict mode";`, `const` works in an ES6 way in io.js but throws a syntax error in Node, though if you don't use `"strict mode";` you continue to get the pre-ES6 non-standard `const` that V8 has provided for a long time.
A minor one is that in io.js, `require('events') === require('events').EventEmitter` while that's not true in Node (where require('events') just gives an object with `EventEmitter` and possibly other properties on i.
