When you ask people to imagine a feature, they're going to imagine the absolute perfect version of the feature, for them personally. A feature that requires no effort to set up or use, provides exactly the information or functionality they want, and never gets in the way of anything else.
Everyone takes for granted how smooth their own workflow is to them, both because they're already used to it, and because they sunk effort into it to make it smooth (via learning or planning), which has probably been forgotten. So, people just take for granted that new features will maintain that acquired smoothness --- unfortunately, they usually don't.
As the author found out, most of the work in building features is often not in the "raw functionality" of the feature, but rather in making something that performs its functions while also not costing the user significant extra mental or physical effort to use.