Here is the full sentence.
From his own experience joining the air force as a young man, Mike had a clue to the riddle. While the rigors of his induction weren’t quite as harrowing as those of West Point, there were notable similarities. The most important were challenges that exceeded current skills. For the first time in their lives, Mike and the other recruits were being asked, on an hourly basis, to do things they couldn’t yet do. -- Source
In this sentence,to me as a beginner, 'The most important' looks incomplete to be used as a subject. I thought it is inverted, but it turned out no.
Could you explain grammatically?