Couldn't comment right away, because I couldn't watch the debate live. (I'm not getting up at or staying up until 3:00 a.m. to watch these crappy candidates.)
I think there were really two debates. For the first 40 minutes or so, Trump was mediocre - about a B- or C level performance. He wasn't great like some commentators have suggested. He stumbled over his words and was often incoherent, and it was clear that he's very new to advocating conservative policy and doesn't know it beyond the most superficial levels. Furthermore, he blew several opportunities, but he also got some good licks in. The train wasn't roaring, but it was at least on the track with the engine running albeit with hesitation. However, as the debate wore on, he completely imploded. He virtually stopped talking issues and spent almost all of his speaking time rambling to defend against attacks Hillary was dishing out or trying to explain away stupid things he has said.
So what went wrong on substance? First, he let Hillary argue that low taxes and inadequate spending led to the financial crisis and Great Recession without any serious rebuttal. I damn near kicked in my TV set when he just let that slide. What kind of ******** argument is that? He should have brought up Bill signing the repeal of Glass-Steagall, which would have allowed him to segue into ripping her sleazy connections with the banking industry. It would have put her on the defensive on an issue on which she has no meaningful defense and would have pleased the Bernie Sanders crowd.
Second, he babbled incoherently when asked how he'd bring jobs back. He should have had a concise answer. "What we're going to do first is make sure the government stops encouraging them to leave. Our corporate tax rate is one of the highest in the world. We need to bring it down to a rate that's more competitive with what other nations impose. Second, we need to roll back some of the more onerous regulations that are suppressing growth, especially Obamacare and the crippling restrictions on domestic energy production that are hurting our workers in places like Pennsylvania and West Virginia. However, we also need to use punitive measures. If a company decides that in spite of our efforts to foster a fairer business climate, it wants to flea to another country and export to the United States, we're going to impose a tax on its goods to make sure it doesn't undercut the businesses that stayed in the US and produced good jobs for the American people."
Third, he was too easy on Hillary on the race/police issue. Though she did it using delicate verbiage, she basically blamed everything on police racism and guns. Trump rightly called for law and order, but he overplayed that and needed to do more. He should have condemned Hillary for presuming the worst of our police officers (which she did), who do everything they can to keep neighborhoods (including black neighborhoods) as safe as possible. He also should have have torn her a new one for rushing to judgment in the various police shootings. Her overt pandering and immaturity on the issue has been disgraceful and very easy to attack. And he NEVER should have gone along with her on any facet of the gun issue. That was just plain dumb. I also would have backpedaled on how crappy black areas are. He has already made his point on that, and everybody knows black areas are mostly bad areas without him celebrating the point. What he should have done is pitch an agenda that promotes job growth in those areas, and he should have promoted school choice - another issue he could hammer Hillary over.
Fourth, big blunder on the foreign policy and cybersecurity issue. Hillary gave what would be a superb, textbook answer for any other politician - but not for her. She had an illegal private e-mail server, which forever makes her the poster child for sloppy cybersecurity. But what did Trump do? He completely neglects Hillary's weakness on the issue and goes off on a rabbit trail questioning whether Russia was involved in the DNC hacks. Probably the biggest blown opportunity in the debate.
Fifth, he wasn't prepared for the income tax and birther issues, which is indefensible. Only an idiot wouldn't see that coming. Even if Holt didn't bring them up, Hillary sure as hell was going to bring them up. He had a few sassy remarks about them (some of which were better than others), but he needed a lot more conciseness and a lot less hem hawing around. He should have had a very clear and well-stated case for why Hillary and her campaign tried to exploit the birther issue.
Other problems that are more related to form than substance. First, he interrupted way too much, and it made him look like a jerk. I think one can interrupt in a debate, but you have to pick your battles and to it very selectively. It seemed like he was talking over her every time she spoke, and most of the time, the interruptions weren't effective. "Where did you find this?," "[not paying taxes] makes me smart," "you've been fighting ISIS your entire adult life," etc. were some of the snarky, stupid lines he tossed in usually through interruptions, and they just just made him look like prick. They did nothing to help his case.
Second, at the risk of being lewd with ladies present in this forum, Trump must have small penis issues like Rubio suggested, because the guy just couldn't resist the urge to blab about his greatness and even worse, the urge to get defensive when his greatness was questioned. How many times did he actively or passively suggest that he had a lot of money? Who cares where he owns property? Who cares if he's "underleveraged?" How many Americans even know what that means? On foreign policy, I damn near spit up my beer when he said NATO started making terrorism a priority when he started talking about it - as if NATO sits by and waits for Trump to talk before setting its priorities. When he caught Hillary in the flip-flop on the TPP, instead of saying that she supported it until she heard him make such a great case against it, why not say she opposed it until Bernie Sanders made such a great case against it? That would have been far more effective, because she can't afford to piss his people off. But instead, he just fed his own delusions of grandeur. "Look at these big hands!!"
Third, he let Hillary set the agenda far too much in the second half. He was starting to get the upper hand on a few things early, and she just started going after him about tax returns, race, and his nutty comments about women, and he just set himself on fire. He could have dismissed her attacks as an irrelevant diversion to keep from talking about real issues (which would have been a true response) and then immediately gotten back on issues. Why not use the anti-woman junk to talk about his maternity leave plan? Why not use them to discuss how proud he is of the hardworking women who work for his business or say something nice about women in his family like his wife, his daughter, etc.? Instead, he had to belabor the point about how mean her attacks where and how he passed on saying the worst things (presumably about Bill's infidelity) because they weren't nice. That was the most painful exchange, but he behaved like that most of the second half of the debate.
Can Trump put out the dumpster fire in the next debates? Most likely, yes. Mitt Romney mopped the floor with Obama in the first debate in 2012, and Obama rebounded well in the next ones and won the election. However, he needs to take them much more seriously than he took this one, because if the same Trump shows up again, he's going to get "schlonged."