*sigh*
There are multiple differences with BBM.
First off, BBM is entirely FREE (even if you don't have text messaging) and works over your data connection. So a person would be able to send as many as they want without worrying about their texting limit.
Secondly, BBM is so tightly integrated into the BlackBerry smartphone that you never have to sign in, it always just works.
Thirdly, your instant messaging "name" is simply your BlackBerry PIN. All BlackBerries have a unique PIN associated with the phone. (Similar to a MAC address for those who know.) You can use a PIN to message another BlackBerry via BBM or by simply emailing them (the PIN would be the address.) All communication via the PIN is free.
Fourthly, one can use a QR barcode to add a contact. For instance, if you were at some social function (say a nightclub) and you met someone with a BlackBerry, they can show you their BarCode and your BBM will automatically request to become "friends" with that BlackBerry and vice versa.
Fifthly, one can use a PIN message (via e-mail or BBM) to send media content, similar to an MMS message, without the space limitation. Instead of compressed audio or pictures, you'd be able to get the full uncompressed version of it.
Sixthly, you can assort contacts into groups, (say "CoWorkers.") If you do, all people in the group will reflect the same information on their BlackBerries. You can administer a group and set privileges as well as allow or deny membership to others. Media and location can be shared with the group without having to upload to each individual member, and you can also send group messages or have conference chats.
There are many other features I could list, but I have to get ready for a long drive so I don't have time now.
The point is, BBM is a hell of a messenger, and honestly I don't know of any that are similar. I do think, however, that Android phones should incorporate such a feature.