I think they both didn't have the right level of preparation.
Hillary was too prepared, and within the first 15 minutes tried to reframe the debate twice. The first attempt- trying to reframe it about trade- was entirely unsuccessful, and she abandoned it not long after attempting it. The second one, however, (when she mentioned the 2008 financial crisis) allowed Trump to take some massive bait. It was an example of Trump's defensiveness, that imo got him the loss, and Hillary's over-preparedness which made her weaker.
Trump was underprepared; he made a lot of silly statements, like "It's called business", and interrupted like hell. It might work against Jeb, but Hillary is completely different.
What does this show? It shows that both of them have next-to-no actual debate experience, and they both struggle to rebut points made instantly. A good debater
has to be able to rebut points instantly, by thinking on their feet, but both of them demonstrated that they can't think on their feet well.
http://qz.com/791320/2016-presidential-debate-how-to-win-a-debate-according-to-the-world-champion-harvard-debate-team/I recommend reading this article. In summary, it has these 3 key points when it comes to presidential debates.
responsiveness, clarity, and authenticity.
responsiveness- the most important part, and Hillary beat Trump in this regard. While she couldn't immediately rebut points well, she did manage to reframe the debate several times to her advantage, where Trump generally took the bait.
clarity- As far as how clear they were, they won in different regards. To your average American, Trump won. He uses the vocabulary of an 8 year old, which is actually smart, because it allows him to connect to as many Americans as possible, and is perhaps why he is managing to gain loads of blue-collar working class votes. Hillary is more clear, in outlining her policies. She gives a much more clear direction of "Point A goes to Point B goes to Point C", up until Z. Donald, on the other hand, makes the vague assertion of "She didn't make this change for 30 years; that means I am your lord and saviour!" From an intellectual standpoint, Clinton beat Trump.
authenticity- While Trump suffered a little from the whole tax-returns debacle of his, he managed to refute the claims against him quickly by diverting it to Hillary's clear opaque attitude. In general, Clinton: isn't very authentic; likes to flip-flop; has been involved in numerous scandals, and so on & so forth.
However, I feel like because of Clinton's policies+policy clarity+policy choice, that she makes a better candidate than The Donald right now, despite her unreliable tendencies.