Well put. Karaites are rare. There's probably only about 50, 000 worldwide, if that. They are generally accepted as Jews, although outsiders may not see them as Jews (Nazis didn't, interestingly enough). There are a couple out there.
Another interesting example is that the Greek Jews use the Jerusalem Talmud, as opposed to the Babylonian Talmud.
But as rmlew said, if you believe Jesus was the son of G-d, or the Messiah, you are a Christian. Which is fine and good, but it's not the same as being a Jew.
Judaism's beliefs are very clearly laid out, and given there aren't that many of us anyways, and it's pretty flexible in terms of orthodoxy, the official branches just basically are classified by degree of orthodoxy.
Please see my responses to rmlew on the same issue.
If I were a chr*stian I'd be very, very confused to learn that being a chr*stian is "fine and good" but that no Jew must ever be one.
Why not just come out and say that Jews are supposed to be Jews and non-Jews are supposed to be Noachides and that chr*stianity and other religions invented by men aren't really that "fine and good" after all?