Rise of Nations was the first game I ever worked on, from the usability and playtest labs at Microsoft. It's also where I met Dave, who I'd later start a company with. I spent a lot of hours behind the glass watching people fail to find their own units.
The scout was the worst offender. A single small unit on a big map, and new players lost it constantly. The fix that stuck was not a bigger sprite or a brighter outline. We gave the scout a dog. A little running companion made the unit read instantly, and people smiled when they found it, which is its own kind of feedback.
We spent as much time on sound as on anything you could see. Brian Reynolds described the two sounds he cared about most: the solid, meaty thunk of a click that registered, and the short screech of not enough resources. You could play with your eyes on the battle and your ears on the economy.
The learn-as-you-play mode was voiced by Boadicea, an in-game unit who walked you through the basics while you played. We refused to call it a tutorial. Tutorial sounds like homework. We also made it very clear when the voice of god stopped and you were on your own, because the worst thing a teaching mode can do is end without telling you.
Twenty-odd years later someone brought Boadicea to life again, and I saw her in a light I never imagined. The decision from that project I still apply: when something is hard to see, ask whether it's hard to hear, or hard to love, before you make it bigger.