At Compelling Science Fiction, we have two main goals: to find, publish, and promote the best science fiction stories, and to support and encourage the authors who write them. We currently pay 8 cents/word for accepted stories (we are considered a SFWA Qualifying Market).
The stories we accept almost always:
The ‘concept’ referenced in point (1) can be anything from new engineering ideas to novel social constructs to abstractions that help illustrate basic properties of reality. Point (2) is far more subjective, although as long as your story consistently moves forward it will probably be engaging. Point (3) is hard to attain without some knowledge of basic science/engineering; we encourage you to learn as much as you can about your subject matter. If your story is very strong in one of the above areas, we will sometimes accept weakness in the other two. Ultimately we are looking for science fiction that is entertaining, scientifically plausible, self-consistent, and technically detailed when appropriate. We have a preference for positive stories, to act as a counterweight to the currently popular post-apocalyptic genre. We certainly won't dismiss a well-written dystopian future, but we have a bias toward stories that depict technological advancement rather than decay. We do not publish fantasy stories (although we enjoy them!). For more information about what we look for, please reference my plausible science fiction blog post.
If you have a story that doesn't fit these guidelines, please visit ralan.com for a wonderfully curated list of other professional markets that may fit your story better!