Something interesting has to happen in the first 20 pages or I'm out. Doesn't need to be action-adventure, but something engaging has to happen. Crazy situation, interesting characters, bizarre setting... something.
I prefer SF that's either a complete mindf*ck a la Philip K. Dick and some of the New Wave writers, or social SF with a whopping-big what if in the middle... but preferably both. SF as metaphor is big for me. Even if it's not the most original metaphor, it should be there. It tells me the writer at least tried to think about their story and setting in more depth than just 'oh, neat, 'splosions'. Interesting twists and ideas that at least try to cover new ground, or look at an old idea in a new way. 'What if aliens are real and they invade' could be utterly boring and trite or it could be spectacular is done differently than the mindless and heartless action that's so typical. Other varieties can be fun from time-to-time, of course, but that's the stuff that really gets me going.
But it's gotta be interesting what ifs that are explored without bogging down in excruciatingly dull details. Huge lists of equipment is a dead stopper for me. Tell me there's a backpack with some gear or a toolbox about and that's good enough. If the writer's so dull as to list out a dozen or more items, or go into painfully boring detail about the history and physics of the equipment and tech, then I'm out. I don't read SF to glimpse hypothetical technical manuals or dull history lessons about the setting. If it's relevant to the story the writer should be able to include it in dialogue or quick exposition without need for length tracks about this scientists and that discovery. A sentence or two is fine, more than a paragraph makes my eyes roll back in my head. More than that and it's permanent consignment to the bin and swearing off the writer.