Actually I think Lefty has been pretty helpful in the sense the metagame of interviewing is important. It isn't necessarily how much you know but rather in what manner you learn. This is so important (not just in trading) because in many situations at a new job, your previous knowledge isn't quite what is so useful, but rather what is useful is:
a) the speed at which you obtain the new knowledge that your new position provides that is required for the job
b) your potential to create new value; really what we are talking about here is raw genius/creativity
Now it is true that there is some correlation between just the depth of your previous knowledge and your potential for a) at least, since to some degree your previous knowledge is based on time and a). But really what matters most is a) and b) directly. The whole point isn't to see if you can answer "depth of knowledge question # 10" or not, it is HOW you answer.
In addition to that, there is the whole playing the metagame of the interview. Let me give you some real-life examples:
Awhile ago I applied for a position as a programmer for cnnsi.com, a website about sports (an online version of Sports illustrated). I wasn't particularly into sports other than watching the playoffs/Superbowl in football each year, ie I was a casual viewer at most, not a sports addict/statistician. My recruiter told me (incorrectly) it didn't matter.
So I interviewed with them, and was pretty impressed with the setup, etc. The interview itself seemed fairly easy, nothing too tough technical wise. Then we break for lunch. It is me and like 3 other of them. The whole lunch period, I was pretty silent because they were talking back and forth about all these stats (that I hadn't heard of), not just RBI but you know the other 10-30 baseball/football/basketball stats and which up-and-coming players were the most exciting. I was clueless at the time and sort of thought they were just kind of ignoring me (not in a mean way), but just were good friends and were having a good time chatting together.
Anyway, I could sort of tell that I just wasn't into sports enough to really be a good candidate for the job. That is exactly how they felt-- I didn't get the job and the reason given was I was a great candidate personality and technical-wise for them, but I didn't have a passion for sports.
Btw I didn't realize this either during the interview or even after it for a while; I was pretty clueless. I just realized it months later.
At yet another company, they had some other interesting meta-interview techniques. They basically had me interview with about 10 people over 12 straight hours during the day, including answering questions in between mouthfuls at lunch. They make people work pretty hard so were actually testing stamina in addition to technical skills.
At my own programming group, we have a policy when interviewing people that we start with easy questions and increase the difficulty to 1 level more than they are capable of answering. This tells us both the interviewees current knowledge maximum as well as how do they deal with challenges. Sometimes we are nice and even tell them that up-front before starting the questions, as in "don't worry about getting our questions right, no matter who you are we always will purposely ask you questions that are above your level just to get a better sense of your thought process."
Anyhow I have seen repeatedly in my personal life how a) learning speed b) creativity c) various other meta-game interview techniques are in fact commonly used by many high-end groups when interviewing candidates. An interesting corollary is flipping this around, as someone looking for a position, if a group doesn't ask you these kinds of questions that is sometimes a bad sign, at least if you are looking to join a group that operates at the highest level in its field.
Hope that helps,
-Taric