I'm assuming you mean science writing for the general public rather than scientific journal writing. The key is striking a balance between being authoritative and conversational. Writers like Neil DeGrasse Tyson, Brian Greene, Neal Shubin, and Jerry Coyne are excellent examples. They have a warm, inviting style to their writing that conveys their enthusiasm for the science. Good science writing shouldn't feel like a lecture - it should feel like you're chatting over coffee about particle physics.
Organization is also very important. It's easy to go off on tangents when writing science articles, since there are so many concepts and principles that have to be explained. Too many tangents and the reader forgets what the main point is. Too few explanations, however, and the reader gets lost in jargon. The right balance here is key.