I started learning gemara based on what Art Scroll had released and what was on sale. (This was before there was an Art Scroll sha"s...) My second masechta was Megillah (on sale for $14.95!) and was right around the time that R' Yerachmiel Fried moved to Dallas and began creating what would evolve into DATA (Dallas Area Torah Association). R' Fried asked me what I was learning and where I was holding; I was proud to say "Megillah, daf zayin or so" (I had barely learned the word "daf" and I felt so wise.) R' Fried said, "Oh! The 'ein beins'"; I was floored... it was my first contact with a talmid chacham of that order -- one who would know the topic off the top of his head when hearing any random daf in sha"s.
The "ein bein's" are a series of mishnayos (and subsequent discussions) contrasting the differences of various holidays and seasons. "ein bein A l'B tov ele X bilvad "/there is no difference between A and B except X only. One of those says there is no difference between Shabbos and Yom Kippur except that flagrant/intentional violations of Shabbos are punishable by death, whereas the same violation on Yom Kippur only incurs an eternal excommunication of that dimension of the soul. (Difficult to consider eternal excommunication as an "only", but there you have it.)
You might be thinking, "Hey... there are lots of other differences. How about the fact that you can't eat on Yom Kippur and you can't fast on Shabbos!?" True enough; the "ein bein's" are meant both to focus attention on one dimension of the comparison and open discussion on others. None that less, you can't (as the English expression goes) compare apples to oranges. The fact that A and B are in one "ein bein" phrase means that are, in some sense, in a single category. Among the "ein bein's" is one that contrasts Shabbos with Yom Kippur, and another that contrasts Shabbos with Yom Tov. One may wonder how Yom Tov and Yom Kippur contrast. That is, should you think of Yom Kippur as a slightly relaxed Shabbos or as a super uptight Yom Tov?
It turns out that Yom Kippur is a super uptight Yom Tov. Siman 611, syef 2 notes that any malacha (including carrying) that is forbidden on Shabbos is also forbidden Yom Kippur and that anything that is muktzeh on Shabbos is also muktzeh on Yom Kippur. So what's left? A big one, it turns out.
If someone flagrantly, publicly, and without shame eats pork, that's bad and he is deserving of lashes, but you can still eat his cooking. If someone flagrantly, publicly, and without shame violates Shabbos, though, he is considered to have denied the entire Torah and is treated in many ways as a goy; you can't even eat anything he cooks because of bishul akum. (We are not discussing a tinok sh'nishba... that's a different shmues.) How about someone who flagrantly, publicly, and without shame violates Yom Kippur? You can eat his cooking (not on Yom Kippur, of course). The difference is that for all it's stringencies, Yom Kippur is just another mitzvah. Shabbos, on the other hand, is much more than just another mitzvah -- keeping Shabbos is testimony that HaShem, the one and only true source of everything, created the world. One who flagrantly, publicly, and without shame violates Shabbos, therefore, is publicly declaring that he doesn't believe in the Creator.
More than the Jews have kept Shabbos, the Shabbos has kept the Jews.