Divergent isn't the kind of word you throw around every day. When was the last time you said, "Hey, we're going over to Andrew's house to be divergent"? Probably never. Because divergent is a slightly weird word. And maybe that's the point. Right off the bat, Roth is telling us that this world is, well, divergent from our own.
Divergent also nicely refers to Tris, who is (a) Divergent and (b) the main character. But why not call this book "Beatrice" or "Dauntless"—those aren't names and words we hear that much, either.. (Apologies to anyone named Beatrice, but your name isn't in the top 100 most popular names.)
Maybe that's because what's really important about Tris isn't that she joins Dauntless, but that she feels Divergent. This leads Tris to what's probably her most common feeling: that she's not sure where she fits in. That's a common feeling with a lot of people, especially young adults.
Now, we don't want to say that being "Divergent" is the same thing as being a teenager, but, well, we can at least say that being a young adult is like being Divergent because they're both tough with a capital ugh.
Of course the fact that she's divergent is actually Tris's greatest asset. It's what enables her to think outside the box and avert Jeanine's attempt to off any and all Abnegation members she can find. So maybe, just maybe, Divergent is Roth's way of reminding us that we're at our best when we refuse to be categorized. We should celebrate our contradictions, rather than snuff them out.