I am 24 years old and I am looking for a new job. I have a college degree, I am clean, well-dressed, articulate and knowledgeable about the fields to which I am applying. Unfortunately, I have been passed over recently for two different jobs due to lack of office experience. Perhaps it has to do with the current economy combined with the fact that I live in New York City.. I often see job listings that say, for example, "Candidate must have graduated from top-tier college with high GPA, must have 2-3 years office experience" for a receptionist position!

My lack of office experience, in the face of a hiring manager, makes me feel like a dog that hasn't been housebroken. Why is prior office experience so important, and what can I say during an interview to mitigate my lack of it?

I'm going to stick my neck out here, and may be shot down.
I don't think it's so bad to white lie on a job application. I started that way, saying I worked for my father (I did, for 3 days, but I made it sound like it was summers and every day after school). He had his own business, but you get the idea.
I filled in gaps too. Easily described because what was around it was true.
Creativity is needed. You really CAN turn '2 years required office experience' into something.
I was once told by an employment agency employee that they assume that resumes are padded and that salaries are inflated by an average of 10%.

Aside from your resume, it is GOOD to sell yourself in a cover letter, and say all you said here.
Add punctual, no time constraints or commitments, and willing to go the extra mile for advancement within one company. Employers can't ask if you smoke so if you don't, say so (smokers seem to be outside a lot... ). If a receptionist position, say what phone systems you know, and if you don't know any, learn one and mention it. Say you can type a business letter while composing it and not have to pause. That helps fill out those 2 years they want that you white lie about. It's all part of the business hustle.

With the economy the way it is, training can be cost intensive. Thus, hiring someone with previous experience cuts down on this expense.

Just my 2 cents.