As the extreme left and right edges of Torah observance continue to move in their respective directions, it becomes more and more important to define exactly what Orthodoxy really is. After all, there is no patent on the term, or other leading words. For example, for years the Conservatives called themselves "traditional" even as they were ripping up every Jewish tradition they could vote on. Nowadays, whether it's YCT on the left with female rabbis or the Neturei Karta on the right with that hate-filled dogma, it's important to try and define what puts a person or institution within "the pale" (a favourite expression of Rav Benjy Hecht).
I would suggest that the following mishnah from Avos supplies us with the definition:
Rabbi Elazar for Modi'in says: He who profanes sacred things, degrades the festivals, puts his fellow man to shame, violates the covenant of Avraham Avinu, and who interprets the Torah contrary to the halacha, even though he is leared in Torah and possesses good deeds, he has no share in the World to Come." (Avos 3:11)
The Tiferes Yisrael on this mishnah notes that there are five levels of non-believers referred to:
1) People who deny God's existence. Since there is no good, there is no holiness, everything is profane so sacred things are not different than anything else in their eyes.
2) People who believe in God but deny that He created the universe. As a result, they ignore the Festivals because they were instituted to remind us of how God intervened in the natural course of the world to take us out of Egypt.
3) People who believe God created the world but deny that man was created b'tzelem Elokim. As a result, a human being is nothing special in their eyes. We are the great-grandchildren of apes and therfore concepts of respect and dignity are quite limited.
4) People who believe God created the world and man but deny God's special relationship with Avraham Avinu. For them, we are not am segulah but rather just one more ethnic group amongst the many of the world.
5) People who believe God created the world and man, and that He has a special relationship with us but deny the validity of the Oral Law. For them, it is all an invention of "the rabbis" and is therefore not authoritative. Anyone can read the Written Law and interpret it for himself.
Using these five levels, one can come to a decent definition of Orthodoxy. The first three are, of course, easy. Calling oneself an Orthodox Jew while denying God's existence, or His creation of the world, or that human beings are not just well-dressed apes but are a special creation separate from animals because of the souls we possess, is foolish.
But the 4th and 5th categories are a little trickier. There are people who called themselves Orthodox, and who will admit the first three categories but will then say that in light of Biblical scholarship and archaeological finds that none of the stories in the Torah actually happened but were the product of much later authors. For them we are a tenacious people who have survived the vicissitudes of history but that the special history-changing moment at Sinai 3329 years ago never happened.
Similarly there are those who don't so much deny the existence of the Oral Law but who feel that they are quite capable of playing with it to their heart's content so that they can create a Judaism that meets their personal standards. This kind of abuse of the halachic methodology is not unique to either end of the Orthodox spectrum. Whether it's inventing the concept of the female rabbi or teaching that those who disagree with you are evil and the cause of everything bad in the world, it is unacceptable. There is a mesorah and it cannot be hijacked by people with agendae.
Thus it seems to me the best defintion of Orthodoxy is conforming to the five conditions of Rabbi Elazar without trying to invent rationalizations to get around them.