Questioning the end-user value of technical decisions (or more broadly, aversion to complacency). It's easy to give merit to past decisions: "this feature is obviously necessary otherwise it wouldn't be there, even though I can't understand how this is useful to anyone". I believe a stronger engineer won't let this slide, especially if the feature is proving painful to support.

I've seen others (and have myself) supported costly features of questionable value. Sometimes a few conversations with product managers and customers have resulted in us entirely dropping a feature.