。
Almost all the leaves have fallen.
Literal
Tree-[possessive-の]-leaves [topic-は] almost all fell-[completely-しまった].
ほとんど ('almost') is added before みんな to soften 'all' to 'almost all.' A clean illustration of how a single quantity adverb can adjust the scope of a statement without rebuilding the syntax. The combination ほとんどみんな is a natural and very common one in casual Japanese.