Compared to the previous experience, the new one is richer, for sure! The toolbars are closer to the message input field which is a big plus: immediate access is a net gain. There is not this feeling of editing an article: the discussion experience is different, so there is no possible confusion. Great!
Do you plan to add this very convenient big blue button at the top of all talk pages as the default experience? Not seeing where to start a new topic is a source of confusion to newcomers. Here, the button may help, even if it could be more visible. French Wikipedia has have a big button for new message as a best practice for people who interact with newcomers (mentors, patrollers...). I guess that's the next step? :)
When you start a new conversation and use a big monitor, you may not notice that something is happening at the bottom of the page. I was confused by this.
I expected to get a modal, or a new window opening. After all, it is a new message: you don't really need to have the context of previous messages posted on the page there. I use Flow on my volunteer talk page, and it allows me to have a separate page for message creation, with just two fields: one for the topic and one for the message. I think this reduces the confusion by removing the context around when a newcomer wants to write me a new message. If the direction goes to keep the new message input like it is in the prototype, I think highlighting the entire input experience (for instance with a grey background), in order to isolate the new message from previous contents, would avoid confusion like "oh, I'm replying to the message at the bottom of the page".
Speaking of which, seeing that you're posting at the bottom of the page may confuse some people, since it is not a natural experience.
It is possible to add a new message without a topic. I thought having a topic was mandatory. I think it should be, to avoid confusion between messages.
Will it be possible to remove the preloaded wikitext, and just add it when the message is posted? Preloads being accidentally broken by newcomers happen a lot.
How will you address cases where experienced users add a lot of colorful, full-caps messages on the EditWarning, to tell newcomers (mainly newcomers) not to do this or not to do that? Would it be possible imagine a new topic field with better integrated advice (and no possibility to use colors and caps)?
It seems that the edit summary remains filled with the title you blanked from the topic field.
Thank you for designing and coding this. It is a big, promising step forward!