Going back on this post, I'm going to try to explain how and why I distinguish between Orthodoxy and Conservatism. This is a layman's effort, as I've done limited study on theology and hashkafa, so if you don't want to hear me pontificate, move on along. Orthodoxy requires some mixture and belief and practice. These two elements underpin Orthodox Judaism as we know it. I'm not quite sure what the mix is though.
Is there really a clear, bright line between Left-Wing Modern Orthodox and Right-Wing Conservative? Let's think of the classic differences. Conservative Judaism does not believe that the Oral Torah comes from G-d. But does Orthodoxy? Some branches believe that every single opinion in the Gemara (Talmud) was told to Moshe at Har Sinai. Left-Wing Modern Orthodox, or even Right-Wing Modern Orthodox does not believe that. They believe certain principles were passed down, but they were applied by human decisors. Certainly an element of divine assistance was at play, but the decisions were human. That's precisely why there are disagreements in the first place. Any other position presumes much of the Oral Torah was lost. I remember Rabbi Herschel Schacter making such an argument.
So in reality what's the difference? Both groups believe that most of the Halacha we have today us man-made. In my mind, the lone difference is the willingness to discard precedent. Conservative Judaism is much more willing to ignore precedent and reverse earlier decisions. Orthodoxy takes a much more conservative (with a small 'c') stand and doesn't move too quickly.
So we see in theology the differences are not so clear. In reality, the primary distinctions are practice. Orthodox Judaism requires certain forms of practice that the other branches no longer do.
I'd say the biggest difference between the groups is keeping Shabbos and Kosher. That seems to be the arbitrary line set by society. Halacha recognizes a specific class of people who violate Shabbos openly. But for whatever reason Orthodox society views keeping Shabbos and Kosher as two of the most important mitzvot.
So the line between Orthodox and not-Orthodox is not so clear. It has become more of a societal concept rather than theological. Keeping these two commandments places one in the Orthodox category, and while the theology is important, the way someone is viewed by society is more based on practice than anything else.