Great question. Ask it to n programmers and get n^2 responses. This could easily be the subject for another post or even a book. Just off the top of my head in no particular order:

  - understands the problem at hand before writing any code
  - uses the right tool for the right job
  - follows accepted standards and protocols without sacrificing creativity
  - names variables & functions what they actually are for the next programmer
  - anticipates what could go wrong before relying on a debugger or testing
  - understands the underlying architecture and how to best utilitze it
  - never writes the same code twice
  - never writes in 150 lines that which could be written in 100 lines
  - Poor code: uncommented.  Mediocre: commented.  Good: doesn't need comments.
  - understands the entire code life cycle & writes it to last
  - has pity on the poor soul who has to maintain it & leaves a clue or 2
  - writes flexibly enough to be easily changed before the project is done

I could go on and on, but you get the idea. In general...

A good programmer writes it right, once, in a week.

A mediocre programmer writes it OK, in 2 months, and then futzes with it forever.

A bad programmer never gets it done.

Original thread: http://news.ycombinator.com/item?id=688300