What it takes to be senior dev from junior dev?

Knowledge and experience

The moment you start attacking problems from a purely mathematical standpoint, where the language implementation is a foregone conclusion.

For instance, consider writing an algorithm which ranks poker hands. If you find yourself bogged down in conditional logic, iterating through cards, you're a junior developer.

The moment you realize a histogram solves most of this problem and substantially flattens the effort, you've grown substantially.

Here's a bit of context: http://nsayer.blogspot.com/2007/07/algorithm-for-evaluating-...