Seggy, good post and good thread. I am sorry that you didn't get the offer. As said on here, interviews can be quite fickle. Sometimes it is just one person that you don't click with which results in the negative outcome.
I don't know if this will help cheer you up, but my first interview was with TWA. I absolutely bombed the interview. Did poorly in the Captain board and really bad in the sim. I received the obvious rejection letter a few weeks later, but then USAir called. I spent the next few weeks preparing like mad. Bought 5 hours of sim time in a funky contraption that an old airline pilot had set up in Oakland, even an hour in a 707 sim across the Bay. Wrote down questions and answers for just about every conceivable HR question. The outcome of that interview was far better. The primary reason is that I learned from the first one and applied what I learned to the second one.
Interviewing is like anything else, it is something you will improve on with experience. It is also something you can improve on with practice. I would highly recommend the services of an interview coach. There are some really good aviation specific ones out there.
In this modern age of HR questions it is important to understand the psychology behind the questions and the answer they are looking for. Once you do it's like a revalation, the correct answer becomes crystal clear.
While the Delta interview didn't go well, use it as a stepping stone to improve for the next one. I'm sure there will be a next one and I'm sure you'll be successful.
Typhoonpilot