In today's business world, most communication is done through writing, whether it's via email or via instant messenger, or if you use Basecamp or Backpack or whatever. You have to write things down, because instant messaging and email are so easy and fast. I think it's really important—especially these days—that people should be able to express themselves through writing.
If they can't quite formulate sentences or explain things clearly in writing, it's going to be a problem. Because communication is really the key to everything here. You need to be able to be clear about what you mean and what your intentions are. If someone can't write, that's going to be a problem down the road, especially for teams that are geographically spread out. [...] So writing is very important; that's how we communicate almost 90 percent of the time.
I should add that reading is the complementary, equally important skill.