Sorry that I don't have references but here's what I can recall from the various reports I've heard, seen or read.
"What is the Benghazi scandal"
It's that the administration is refusing to disclose what really happened. Who made what decisions and when? They've intimidated state dept./CIA employees into keeping silent. What was removed from the original report before the one that was officially released? Who removed those details? There's just too much evasion (or appearance of evasion) if there really is nothing of interest to be found.
Even if everything done by the WH administration was perfectly valid, the mere fact that the Obama administration doesn't believe there is any obligation to answer to the people/congress is arrogant and deserving of whatever scrutiny is thrown at them. Why would it have been so hard for Hillary to simply state what she did and didn't do on the night in question? Instead, she acted like a spoiled brat who was insulted that congress had the gaul to inquire into finding out what happened on that night. Especially, when it is so obvious that many details were being left out in the official explanation.
Nobody has ever gotten an answer to where was Obama and what did he know while all this was happening. Obama is notorious for his dislike of meetings so he chooses to get his security info via email-like reports instead. Some believe that he got the report but didn't bother reading it, so he didn't even know Benghazi was happening.
"What makes it a big scandal"
One theory is that the WH admin chose to not defend the embassy because the voters seeing terrorist activity at that time might cost Obama some votes. So the WH chose to let people die instead. Although, in fairness it was probably more like they chose to leave people in a dangerous situation and it blew up in their faces when people were actually killed. Was the WH willing to endanger lives for the sake of their election? How high up did this decision go?
Another possibility, from above is how "engaged" in doing his job is Obama really. Did he get the security report? Did he read it? Did he do anything about it?
Another issue is the administration intimidating employees into not talking to congress. You can't have checks and balances if people are intimidated into silence.
"Why is it making major news now"
Because congress finally found a couple of people willing to talk. After their testimony, the WH has now gone on to rev. 4 or 5 of their version of what happened. Sorry, I don't remember which WH official testified after Hicks last week but I remember him saying something like what follows that I believe will become infamous "It is not the State Departments job to ...[ughhh can't remember]". But it was kind of an admission that they knew more than they've already admitted to knowing.
Anyways, while the MSM is trying to sweep this story under the carpet, you can see that there's all kinds of side stories and theories that actually make this quite an interesting investigation. Many of the pundits strongly believe that this story is going to take some big-shot down, but nobody knows who yet.