The unforgettable Rebbetzin Braunstein A”H once asked this question to a large audience.
Everyone answered more or less along the lines of these posts.
She then said “As someone who deals with all sorts of issues after marriage, I can definitely tell you that the most important thing to look for and ask about in any shidduch – boy or girl – is: “Is this person a happy person? Is the family a happy, optimistic one?”
What great advice!