MVPs should have deep technical knowledge, and should share that knowledge. 20 years ago I was very active on Usenet, even writing a book about it, and wrote a newsreader as a sample app in another book. Yet by the time this century dawned I had nothing to do with Usenet any more. I was awarded MVP because of my books, blogging, public speaking, founding user groups, and so on.
I advise those who want to be an MVP to do two things. First, just start doing what it is you believe MVPs do. Blog, tweet, give sessions, volunteer. In person, online, by writing code, writing words, saying words, recording videos, whatever it is that lights you up. Share your knowledge. And for sure, answering questions on SO or other SE sites "counts". Second, contact an MVP who has come to know you (over some months) and ask if they think you're eligible. They will let you know for sure. And if they think you are, they can nominate you. I've done it before and am about to do it again.
More at http://www.gregcons.com/KateBlog/WhatIsAnMVP.aspx