In this poem, the speaker considers the various possible outcomes of a dream that has been delayed or put off, and he uses a series of similes followed by one, final metaphor.  Because a metaphor is generally thought to be more forceful than a simile (as it claims that something is something else, not that something is only like something else), and because the final line is both italicized (and thus emphasized in another way) as well as stands alone in the poem (when the only other line that stands alone is the initial question), I believe that it is the truest response to the question: a dream deferred will eventually explode, causing great damage to many. 

The speaker considers whether the deferred dream will dry up, fester and ooze, stink, crust over, or sag.  These are inconvenient, perhaps even gross or obnoxious.  However, the final metaphor compares the dream that's been put off to a bomb, something that "explode[s]."  Something that explodes isn't merely inconvenient; it doesn't just affect one person or a few.  A bomb affects many, and it does great damage.  If a dream, such as the dream of racial equality in the United States, is delayed and put off and disregarded for long enough, the speaker seems to imply, the consequence will be disastrous, and not just for the dreamer, but for everyone.