Solutions, features, and CompatibilityLevel

I recently came across an issue where certain feature definitions were not available in 2010 mode sites in one farm, but they were in another. It turned out that the parameters supplied to Install-SPSolution are the reason. Essentially, the difference was specifying -CompatibilityLevel {14,15} vs specifying nothing. I found this article to be very helpful: Planning Deployment of Farm Solutions for SharePoint 2013