Said in a frustrated, sarcastic tone, it means one thing. It'd mean something different as a two-word caption in a Neil Gaiman comic; there, it would seem something, somehow, poetic.
It could be wistful, sad at something lost. It could be hopeful, looking towards a chance to get something, to reach a goal. It could be cheerleading, rah-rah-ing to yourself or another "You can do it!" It could be matter-of-fact, a statement: Wishing is a thing I do.
So...when you hear "I wish," what do you really hear?