Anything can be compelling (the opportunity to learn a new skill or to stretch an existing one, the opportunity to innovate or to optimize, even solving old problems but tailoring for a new community), it all depends on the person. What's compelling about a specific problem set may flip my switch but be completely boring to someone else. Just be honest about what you're trying to accomplish (ie. communicate the needs of the stakeholders the engineer will serve) and it will resonate with the right person.

It speaks so well of you and your corporate culture that you're asking the community you'd like to reach, without making assumptions. In my opinion, you list table stakes. Excellence in attributes like flexibility and benefits are base expectations for qualified engineers with choices. What matters most to me when making a decision on how to spend my time: 1) the problems I get to solve, and 2) the people I get to work with in the process of solving them. If you highlight what is compelling about those two things then I think the right people will get excited and connect. Good luck and I hope you find great people/position fits!

These are oldies but they might be new to some :-) 1) Q: How many programmers does it take to change a lightbulb? A: None. It's a hardware problem. 2) Q: Pat starts the day with 27 bugs. Pat works very hard all day and fixes 5 bugs. How many bugs does Pat now have? A: 43.