(And of course, you should have your portfolio/github/etc. That's a given.)
- You're passionate about being a software engineer. This could manifest itself as side projects, or simply showing that you were passionate about a work project and went above and beyond.
- You've had broad experience, and understand many areas, not just one narrow area. This is less about specific programming languages as broader technology areas. For instance, have you worked on performance tuning? Can you explain to me how a network works in excruciating detail?
- You have a formal CS education. Some may not care about this, but it's a good indication that you have solid fundamentals, and understand things like big O notation. If you're a new grad, getting good grades is important too.
- Have you worked at another startup? If so, I want to know about it - it's an indicator you understand what working at a startup entails.
2. Your own projects, using interesting/relevant technologies.
3. Proof that you get Stuff Done (ie. your own projects).
4. That you can wear multiple hats.