Wave-2 sources state it most forcefully: the base IDEA — not title/thumbnail/quality/budget — is the #1 factor; 'if the idea is weak, nothing else can save the video.' Idea decides 100 vs a million views.
The single most important factor for determining whether a video blows up is not the title, not the thumbnail, not even the video quality itself, but the base idea.
Real outlier videos that demonstrate this principle — play any in-app.