A lot of college seniors think it's sufficient to be presentable and talk about their GPA and school activities -- in effect, to recite their resumes. It's not. You're competing against a lot of other students who went to similar schools and got similar grades; if you got an interview, you've already been screened for those things. The purpose of the interview isn't to screen for that; it's to get a sense of your personality.
Put yourself in the interviewer's shoes. Unless his job is in Personnel, he was probably assigned this task against his wishes. He's most likely been asked to screen interviewees for his department, so he'll have a lot of contact with whomever he helps hire. Uppermost in his mind is whether you're someone he'd want to spend a lot of time with. So one of your primary goals is getting him to like you. (No one has ever wanted to hang out with someone because of his GPA.)
One or two self-deprecating comments go a long way in that regard. You just can't criticize yourself for anything which would impinge on your job performance. ("Frankly, I'm dumb as a box of rocks, and lazy to boot." Or, "I have this little problem with ethics, but I think I've put it behind me."). Just be self-critical about something entirely irrelevant to your potential job performance.
If you're being interviewed by a large corporation, your interviewer may be talking to as many as 16 students that day. By 5 PM he's going to be bleary-eyed and will have a hard time even remembering all the interviewees. So, you have to make yourself memorable. (Yet another resume with lacrosse and Amnesty International and summer jobs lifeguarding and a 3.8 GPA is by itself not memorable.)
So say something that only you can say. There must be something unique about you that you can somehow relate to the job you would do for the company.
You also have to show that you don't have any hidden psychological issues which might emerge later on. So mention that you work well with people, and that you take rejection in stride (in most of life, let alone a job, you will lose before you win, and nobody wants to deal with someone who might crack).
I was recently asked by a friend to give his son advice on interviewing for an investment banking job. The son is smart, hard-working, tough, and a very decent guy. He also happens to be somewhat nerdy-looking. Among the things I told him to say was, "I take rejection well. Hey, when you're as geeky-looking as me, you get used to it." I have no idea whether he used that line, but if he did -- and if he said it semi-brashly, it's not a line you want to deliver hesitantly -- it would have worked for him. He would have proved he's not overly sensitive, that he has a self-deprecating sense of humor, and it would have made him memorable. It was a harsh thing for me to tell him -- I'm not even sure my friendship with his father is still intact -- but it was in fact good advice.
You must also do your homework: learn about the company you're interviewing with and what makes it successful and outstanding. This flatters your interviewer, who by extension must be successful and outstanding himself to work for such a company; it shows you want to work there; and it shows you're the type of person who does his homework.
When I was at Goldman, I once interviewed an Olympic champion swimmer. We chatted about swimming briefly, then when I asked him why we should hire him, he just said, "I have a record of success." He had nothing to say about why he'd be good at bond trading. I told him that for him to say that was like a beautiful girl walking into room and announcing that she is beautiful: it adds nothing. (In fact, it subtracts.)
You have to think about what the job you're applying for requires both intellectually and psychologically, and come up with examples of how you qualify, even if your life thus far does not include any actual experience at a similar job.
In 1984 I applied for work as a bond trader. I knew that among other things it was hard, pressure-filled work, and that it involved a lot of numbers. I used my LSAT's and GMAT's as proof that I worked well under pressure. I also said I had always worked hard, especially at swimming, then held open my jacket, shrugged and said somewhat disgustedly, "I went as far as this body was going to take me." That comment about my physical limitations didn't reflect on my ability to trade bonds, but did show self-deprecation.
To show my head for numbers, I said that I could remember every track and swimming world record going back to 1967. If the interviewer wasn't a fan of either sport, I asked him to give me three telephone numbers, and said I would recite them back to him at end of interview. (This tends to impress people who don't realize how easy that is to do with mnemonics, and made me stand out from the other interviewees.)
My interviewing went well -- I was offered a job at every major investment bank except First Boston. My career, on the other hand, was mediocre; I basically got out-sociopathed at the office.
So, I've been left with a middling amount of money and a lifelong interest in sociopathy.
In any case, if you're interviewing for a job, think hard about what I've said above.