TinyMCE is a useful tool for formatting content in Drupal. However, whenever a user presses enter while using it, TinyMCE inserts a paragraph break. A paragraph break inserts a line break followed by a blank line. This is very useful when the user WANTS a paragraph break but very frustrating if they just wanted a line break.
Example:
I'm the first paragraph.
I'm the second paragraph.
I'm the first line.
I'm the second line.
To create a line break in TinyMCE, simply hold the Shift key while clicking Enter.