I would propose that expressive descriptors like foreboding and ominous are sometimes more than just subjective reactions, more than just the projection of a listener's emotion onto a pliable medium. Sometimes they are a succinct encapsulation of a theme's structural function. The opening theme of Shostakovich's Tenth Symphony is foreboding, I would argue, not because what it "expresses" in the moment can be nailed down to a specific human emotion (as an isolated expressive element it could be heard to embody depression, brooding anger, dread, any number of things really) but because it forecasts cataclysm. It, like its human emotional counterpart, anticipates a troubled or even catastrophic future. Its foreboding nature is not in its physiognomy, but in its relation to the later musical events in which it is implicated, including the harrowing climax of the first movement, the desolation of its coda, and the theme's return as a foil to forward-looking elements in the third movement. And if indeed foreboding is a part of its structural function in the symphony's opening, one won't know it until what it forebodes is manifest — not before one understands its role in the thematic processes of the entire symphony.
Anyway, the opening statement of Shostakovich's Tenth is one of my favorite examples of musical foreboding.
Edit: On second thought, there is another viable interpretation of its expressive-structural function. That opening could be a threat, because threats also forecast unwelcome future events. It depends, I suppose, on whether one chooses to identify with the symphony's voice in the third person or in the first.
