I just don't see it, really. This whole email thing seems, to me at least, to be little more than a push-and-pull between State and the FBI, specifically with how they handle and disseminate potentially sensitive information. So, why this long investigation if there's not really anything criminal there? Well, politics is partially responsible, as usual, where the FBI has to cover all their bases to ensure that their independence from the White House remains relatively unquestioned, but also to say "Hey, State, you did some things/are doing some things that we aren't particularly pleased with, and when you do that you can expect us to bring out the microscopes."
I view it similarly to the GOP making noise about impeaching Obama for this or that. So, why haven't they when they clearly want to? Because there's not really a crime, it's a push-and-pull between two different branches of government, especially in areas where the division of powers isn't clearly laid out. Those are matters for a court to sift through and then make a determination about whether those actions can continue, not a case for impeachment.