Which things in Debate #1 hurt Trump the most?

[Slander and/or Libel]

Any one of those was probably enough for Trump to lose the debate.  However if I was Kellyanne Conway (which thankfully I am not ), the thing that would concern me most would be how easily "baited" Trump was.

This was my take too. Hillary managed to make her baiting remarks subtle enough that most viewers would think it was obvious what she was doing, but if you're immersed in this stuff, it was completely obvious when she was needling him in the hopes of provoking a reaction, and it worked almost every time.

[Slander and/or Libel]

He should have been on the attack more. He did well when he tied his taxes to Clinton's emails, but seemed to back down in every argument after that.

So, according to you, his only fault was not attacking Clinton more. Not anything listed by the OP. Well, I guess that tells everyone here enough about yourself.

All of those gaffes arose because he allowed Clinton to frame the debate, when he ought to have brushed off her attacks and focused on issues of substance, on which she is consistently incompetent. It is asinine that the moderator focused on whether or not a private citizen was sincere in his prescient opposition to the invasion of Iraq, rather than asking the individual who authorized the attack and continued to shill for it for a subsequent ten years. Given even a modicum of preparation Trump ought to have been able to flip all of these attacks against his opponent, but fell into the trap of being on the defensive.

That's such a weird way of framing that.  You're phrasing that like this "private citizen" isn't one of two major-party Presidential candidates, and that it's some sort of irrelevant personal opinion, as opposed to an issue that candidate has used to draw a contrast on judgment.  Trump, not Clinton, is using that as an argument.  Asking Trump this question is having him clarify the logic of his argument (something he hasn't really done to my knowledge); asking Clinton your proposed question would basically be "tell us again how you were wrong" (something she has done, even if you think it was done inadequately).

And besides that, accepting mencken's framing implicitly accepts that Trump did oppose the war in Iraq "presciently", and the best evidence he can mount for that is that Sean Hannity will totally tell you that they had phone calls a couple of times about it.

[Slander and/or Libel]

He should have been on the attack more. He did well when he tied his taxes to Clinton's emails, but seemed to back down in every argument after that.

So, according to you, his only fault was not attacking Clinton more. Not anything listed by the OP. Well, I guess that tells everyone here enough about yourself.

All of those gaffes arose because he allowed Clinton to frame the debate, when he ought to have brushed off her attacks and focused on issues of substance, on which she is consistently incompetent. It is asinine that the moderator focused on whether or not a private citizen was sincere in his prescient opposition to the invasion of Iraq, rather than asking the individual who authorized the attack and continued to shill for it for a subsequent ten years. Given even a modicum of preparation Trump ought to have been able to flip all of these attacks against his opponent, but fell into the trap of being on the defensive.

That's such a weird way of framing that.  You're phrasing that like this "private citizen" isn't one of two major-party Presidential candidates, and that it's some sort of irrelevant personal opinion, as opposed to an issue that candidate has used to draw a contrast on judgment.  Trump, not Clinton, is using that as an argument.  Asking Trump this question is having him clarify the logic of his argument (something he hasn't really done to my knowledge); asking Clinton your proposed question would basically be "tell us again how you were wrong" (something she has done, even if you think it was done inadequately).

And besides that, accepting mencken's framing implicitly accepts that Trump did oppose the war in Iraq "presciently", and the best evidence he can mount for that is that Sean Hannity will totally tell you that they had phone calls a couple of times about it.

Or, you know, the moderator could have done his homework.

That's it? That's the best you could do? He literally said he longed for the days of MacArthur, who would have just gone ahead and marched in. He also said maybe we should wait for the UN. He's all over the place, but he's certainly not in full-throated or even half-throated opposition.