At various times control, power, duration, speed, or reliability have been the Big Thing dominating design efforts. For DeHavilland-Canada, ruggedness and STOL were the big thing when everybody else was concentrating on speed and power. The wonderful Twin Otter was the culmination of that effort.
Actually I meant the very early days, when the engines weren’t even powerful enough to lift their own weights. That’s why the earliest planes were basically kites with tiny engines. Engines had to be completely rethought concerning weight vs horsepower. Before aircraft, the weight of an engine just didn’t matter.