Here are some:

"Having good ideas is most of writing well."

How did you come to this conclusion? Evidence? Citations? Reasoning?

"… how much less ideas mattered in speaking than writing"

Is this based off just the ONE other speaker you mentioned? Any studies? Have you made a personal study of this yourself? Taken notes? I would like to hear some evidence or argument to back this up.

"Being a really good speaker is not merely orthogonal to having good ideas, but in many ways pushes you in the opposite direction."

How so? You have sentences that sort of follow this, but you don't actually explain this statement.

"The way to get the attention of an audience is to give them your full attention"

How do you figure? You've admitted you're a poor public speaker and particularly at this skill, so how are you an expert on how it does work? If you've done some research, I'd love to hear it.

"If you want to engage an audience it's better to start with no more than an outline of what you want to say and ad lib the individual sentences."

How do you know? Have you done this successfully? I have lots of friends who are on the professional speaking circuit (such as it is for tech people -- unpaid, for the hell of it) and I don't know anyone who's an accomplished speaker (except myself) who does it this way. For their part, they think I'm crazy for doing it this way. It works for me but I certainly wouldn't say it's a best practice.

"Actors do… Actors don't face that temptation except in the rare cases…"

Are you an actor? Have you researched acting? Have actor friends? How did you arrive at this conception of how acting works?

"Audiences like to be flattered; they like jokes; they like to be swept off their feet by a vigorous stream of words."

I don't see any proof or further argument to back this statement up. Meanwhile the way it's phrased makes it very clear about what you think you are bad at and probably why you don't believe public speaking has much value.

"As you decrease the intelligence of the audience, being a good speaker is increasingly a matter of being a good bullshitter."

Evidence? Argument?

And, fun: by using a loaded word like "bullshitter," you are relying on emotional reactions instead of appealing to reason or backing up you assertion with facts.

"That's true in writing too of course, but the descent is steeper with talks."

How do you figure?

"Any given person is dumber as a member of an audience than as a reader."

So what you're leading us so delicately to believe is that the audience is perforce dumb and therefore being a good speaker is largely about being a good bullshitter. Do you have any argument to back THIS up?

"Every audience is an incipient mob, and a good speaker uses that."

This just made me laugh.

"Just as a speaker ad libbing can only spend as long thinking about each sentence as it takes to say it, a person hearing a talk can only spend as long thinking about each sentence as it takes to hear it."

So you're saying that you have proof that in a conversation, the listener's entire brain is taken up with listening to each individual sentence, and not thinking about things that came out of the talker's mouth 30 seconds ago? Or, we know this cannot possibly be true in regular conversation, but you have proof it is true in an audience/public speaking relationship?

Also, here you create a false dichotomy only to knock it down: The only good way to speak is to create an outline then ad lib. If you ad lib, you can only think about each sentence as it leaves your mouth. Therefore, you cannot be thinking about what you're saying. Because of course, if you ad lib, you cannot practice or rehearse, because that would be the same as reading…?

"So are talks useless? They're certainly inferior to the written word as a source of ideas."

As jeffdavis pointed out, this statement is actually totally unsupported. You didn't actually address the communicative value of a talk at any point in this essay, you talked about things around (one might say orthogonal) to the value -- e.g. the audience is a mob, bullshitting, ad libbing, getting the audience attention, and some statements about how you can only think of a sentence while you're saying it or hearing it.

I would love to see it if you do have an argument for saying that talks are inferior to the written word, because as you are probably aware, there is a lot of evidence that written communication is inferior to verbal communication -- lower persuasion, higher misunderstandings, more projection on part of the reader, lower empathy, requiring much MORE written communication for the same level of understanding as would be reached by speaking.

"It's probably no coincidence that so many famous speakers are described as motivational speakers. That may be what public speaking is really for. It's probably what it was originally for."

This one is particularly interesting because, of course, the art of rhetoric dates back to the Greeks and no less than Artistotle himself wrote a scroll on the many, many uses of speaking, and how to do it, and how to achieve all kinds of different effects.

It's hard to believe that someone as smart as yourself would make such statements about the value of public speaking without even mentioning any of the prior art (e.g. The Art of Rhetoric by Aristotle, or any of the later thinkers - Francis Bacon, etc).