Well, there are a lot of different kinds of Jewishness. From a religious standpoint, one would not compare a uber-strict Fundamentalist Christian to a person who goes to church once a year on Christmas. The same of course is true with any religion, including Judaism.
Here's a chart that explains the differences between the officially accepted branches of Judaism:
Then of course you have the surprisingly popular Kabbalism...which, well...I'll just leave this link here because I don't have time to write that much at the moment:
http://en.wikipedia.org/wiki/Kabbalah
Then you have Jewish people who aren't religious at all but are defined as such by virtue of genetics alone.
To lump all Jews into the same category is both a hasty generalization and association fallacy. Just because people share one characteristic, especially such a loosely-defined characteristic, it does not follow that they would necessarily share other characteristics.
Example:
P1: My ex-husband was an accountant.
P2: My ex-husband was also obsessive compulsive.
C: Therefore, all accountants are obsessive compulsive.
This makes as much sense as any form of discrimination. Including
http://en.wikipedia.org/wiki/Antisemitism