I love how this gives product managers the ability to mock-up an app without needing a developer.
But looking at the drag and drop "Code blocks", I imagine the most common use case would be for educational purposes(?)

What would be the main market you're aiming for?
Educational?
Build once, run on multiple platforms?
App builder for non programmers (Designers, PMs, CEOs without CTOs)?

Here's some feedback based on my personal impressions from playing around a bit with the UI:
(All on Chrome 48.0.2564.116, Windows 7 64bit).

1. I love that you can drag and drop UI elements, but personally I've never been much of a fan of drag and drop code blocks elements.
This would probably be the #1 issue preventing me from adopting a product like this.
I find it more confusing and more complicated than writing simple code, even in a completely new programming language.

2. It took me quite a long time to realize "blocks" was what I need to click on if I wanted to add my own functionality to the UI.

3. I would expect double clicking on a button or right clicking on it (In either the viewer or components tree) would give me a context menu.
Alternatively - take me to the blocks section related to this UI Element (similar to VB).

4. If I've selected an item, I want to be able to delete it by either pressing on "delete" or by moving it off screen. "Delete" stops working if I add a new element and only returns to working if I click on "Components".

5. The "Apps" menu feels quite overwhelming.
What's an aia (a zipped archive of json, xml and properites of course)?
Why do I even need to download it to my computer? Can I edit it on my computer?
Why would I need a "Save As?" option, and why isn't the project saved every time I edit it?
The Keystore options are especially cryptic, and I imagine you'd have to something to educate anyone who hasn't released an app in the past. Also, would expect them as part of a Build/Release section.

6. Context-sensitive help would be a much appreciated feature. Lets say I add a gyroscope element, and start setting up the gyroscope logic. Would be really helpful if I could just hover or click on a question mark to figure out the possible values for xAngularVelocity or units(ms/s? utc?) used for timestamp.

This information is available through the right click menu, however:
7. This is the first place in the UI that a right click does anything. After trying to right click in the designer with nothing happening, I was actually conditioned to not even try right clicking in the blocks section.
I also discovered the "Shift" functionality to find a code block using a textual search. Why not have a search option always visible? Felt a bit like an Easter egg.

8. Test menu -> Thunkable live. gives a QR code that's only readable from within the Thunkable app.
To actually download the thunkable app, I have to:
a. click on "Need help finding the thunkable app?"
b. Scroll down until I find the QR code.

Perhaps a better strategy would be: Provide a QR code to app.thunkable.com/test/blabla
Register the thunkable app to handle links that start with app.thunkable.com/test...
If the link is opened in a browser, take the user to download the thunkable app. (Redirect to google play)

Alternatively - just provide another QR code and direct link on "Connect to thunkable live" modal dialog.

Really impressive work!
Knowing first hand how much code it takes to set up an android app that displays the users lat/long in a text view, really impressed by how easy it was to do with Thunkable.