> Simple means getting rid of extra words. Don’t write, “He was very happy” when you can write “He was happy.” You think the word “very” adds something. It doesn’t. Prune your sentences.
This is great advice if you want to write like Hemingway. Most of the great authors are not so sparing with their words, though. And I often find that people who excessively prune their writing end up with quite generic paragraphs that seem like they were written for elementary school children. I've personally found a better tip for writing is to not use two "cheap" words when one "expensive" one will convey your meaning more fully. To use your example, "He was very happy" doesn't convey as much feeling as, "He was ecstatic". But depending on what you want to say, "He was restlessly giddy with barely contained joy" could be even better.
Most users of written English are not the Great Authors, though, nor are they writing literature. Hemingway is a much more appropriate model for business communication than is, say, Wallace. Though for attention-grabbing copy, you’d probably want Morrison’s or Nabokov’s dynamism. And if you’re code-golfing, nothing beats a Borges, Chiang, or Cusk.