Background: The state of the art in software engineering consists of a myriad of contributions and the gaps between them; it is difficult to characterize. Questions: In order to help understanding the state of the art, can we identify gaps in our knowledge that are at a very general, widely relevant level? Which research directions do these gaps suggest? Method: 54 expert interviews with senior members of the ICSE community, evaluated qualitatively using elements of Grounded Theory Methodology. Results: Our understanding of complexity, of good-enoughness, and of developers' strengths is underdeveloped. Some other relevant factors' relevance is apparently not clear. Software engineering is not yet an evidence-based discipline. Conclusion: More software engineering research should concern itself with emergence phenomena, with how engineering tradeoffs are made, with the assumptions underlying research works, and with creating certain taxonomies. Such work would also allow software engineering to become more evidence-based.