This question is not about standards of kashrut -- which are of course a serious topic -- but rather the question of "grave sins" for which the death penalty is prescribed, or the consequence of "Kareth", which seems to be either death or worse.
For reference, consider Leviticus 17: http://www.mechon-mamre.org/p/pt/pt0317.htm
And also wikipedia: http://en.wikipedia.org/wiki/Kareth#Offenses_punishable_by_kareth
So my question is about fat and blood in non-kosher meat found in USA supermarkets. I heard that blood is in fact drained by most butchers, although it is probably not done in the way kashering prescribed by the Talmud. I don't know about the fat.
I would like to know the answer to the question in the title. Please include facts about non-kosher meat sold in US supermarkets, as well cite solid halacha in order to make a case one way or the other. I hope that eating non-kosher meat isn't an offense on the level of, say, having sex with a menstruating woman, or murder. But I am concerned.
Also, if you have something to say about Kareth and how you view it, please include that, because I think it is relevant in view of the following fact: the vast majority of Jews have at some point eaten nonkosher meat, violated a sabbath, or did something else deserving Kareth. Upon realizing the gravity of the sin, is one supposed snap and stop doing it until the rest of their life, and even one infraction causes the Kareth punishment? There are some opinions that a certain amount of fasting and praying removes one punishment, but these seem to be mere opinions, and for some people an entire lifetime of this wouldn't be enough to make up for the amount of sin, so the whole Kareth concept seems to be, like the Christian hell concept, to be confusing and scary.