• Is the prose clear? Does it say what you want it to say?
• Is it tight and clean? (Can you cut excess wordage? There's hardly a manuscript alive that wouldn't benefit from cutting.)
• Are there infodumps? (Get rid of them.)
• Tighten! Again!
• If you are using artfully experimental language and style, ask the hard question: Does it work? Or does it just call attention to itself?
• Do your people sound like real people?
• Do your aliens seem alien?
• Are you telling, or showing?

• Can the reader understand what's happening?
• Is the grammar correct?
• Are there spelling mistakes or typos? (Use your spellchecker as a tool, but trust a good dictionary when you're not sure; many spellcheckers have limited vocabularies, and they won't necessarily pick up incorrect usage.)
• Have you given the reader enough detail to picture the scenes clearly?
• Can the reader understand what's happening?