How would an informational interview with the senior partner of a major law firm benefit me?

Mediocre grades, tier 3/4 law school, no journal experience, 3 years of document review/contract attorney work. I am not expecting a job offer or even a job interview.

What can I expect from this meeting?

What kinds of things would the senior partner tell me?

Are the huge firms interested in current practicing attorneys if they only hire from law school?

You can expect to get out of the meeting what you put into it. Leading with the negatives on your resume will not help at all.

What kind of law do you have a passion for? Why did you set up this meeting with this senior partner?

Why not expect to get something, even if it is a contact to someone else who is hiring. The whole point of an informational interview is to figure out what they desire in associates/partners and how you can prepare yourself to be what either they or their peers are looking for.

Have you ever conducted an informational interview? You need to be prepared with what information YOU want to know. Have you googled the terms "informational interview?"

If the senior partner is gregarious or outgoing, etc..., he or she could help you a lot; they might give you substantial information without having to pull teeth to get it. If not, they might not be much help at all.

I assume you set up the informational interview, so the burden is on you to conduct it. Have questions ready. If you did not pick this person because you want to do the same kind of law, this could be a failed informational interview. Get in touch with your motives behind this. Sounds like you are a bit confused about what you want.

So, I'm not sure what point you are at in your life, but the answer to your question is one word: networking.