I'd say bug fixes, tiny typo fixes, documentation additions, unit test additions, performance enhancements, and relatively minor features are all good candidate

I'd say bug fixes, tiny typo fixes, documentation additions, unit test additions, performance enhancements, and relatively minor features are all good candidates. The actual scope of any set of changes that will be quickly accepted can vary wildly depending upon how well defined the project is and what group of individuals is maintaining it. I'd say the defining characteristics would just be easy to read the diff and understand how things are exactly altered and that the benefits are proportional to the risk and time for integration. An adequately written documentation addition for instance would be a easy to check for consistency and structure in one pass and the benefit is generally obvious after reading the diff. Large changes for a first commit can be reasonable, but some extra time has to be expected with some discussion on those changesets.