1) BBM is essentially a text message that is free. This is handy when you're oversees (business execs) for example. You can be in Taiwan and shoot a BBM to your partner in SF without having to worry about some ridiculous charge.
2) Read Notification - BBM will tell you when your message has been read or not (as opposed to text messages)
3) Group Chat - BBM does this really well - very easy to text and have conversations with multiple people.
The "killer feature" of BBM tho' isn't any one thing, it's just that it's an incredibly polished, seamless experience, I have on my BB the Y!, MSN and Google clients and they are just clunky as hell compared to BBM. It's an absolute joy to use.
BBM uses data, right? Wouldn't there be outrageous per kilobyte charges? Or does RIM have deals in place to get around that somehow?
It's also compressed and encrypted.
[1]: http://moconews.net/article/419-average-number-of-apps-downl...