The form editor element chooses the first format in the list of the editor's supported formats. This is particularly annoying when someone has an editor that support two formats and you don't get the preferred one: one has plaintext format and expects Moodle Auto Format to be the format chosen in the select box and not HTML (or one has a Markdown editor and expects Markdown to be the one selected, not HTML).

To reproduce: 

• Configure plaintext are as your default editor.
• Go to a forum
• Add a post.

What you get is format html as the format of the new message.

What you should get is Moodle Auto format as the format of the message.

Of course this is IMHO

Maybe a change like this can be confusing so I'll put it on a new version (4.2 maybe).