The discussion revolves around the difference in line break behavior when pressing enter in the Discourse composer. Brandon007 noticed that hitting enter once creates a double line break in one forum but only a single break in another. Moin and NateDhaliwal clarify that this behavior depends on the composer mode—rich-text (WYSIWYG) versus Markdown. Brandon007 realizes the issue was due to not noticing the M↓ or A icon, which switches between modes. Lilly provides a link to the announcement introducing the new composer, explaining the change.
Hmm… are you using the Markdown or the WYSIWYG rich text editor? IIRC the 1st behaviour is that of the WYSIWYG rich text editor, and the 2nd is the Markdown one.