149 votesJeff Schwisow commented
I agree 100% with the comments below that having ENTER instantly send your message is a poor design decision; particularly given that there is a SEND button (the paper airplane icon) below that looks ideal to fill the role. It is somewhat ironic that hitting the ENTER key in this forum adds a return and doesn't post the comment...
Just to re-iterate what a couple of folks have said below...SHIFT + ENTER does add a return (skips to the next line) to a message.
Additionally, (and this is pretty cleverly hidden from view) using the keyboard shortcut SHIFT + CONTROL + X opens what they refer to as the expanded compose box. This box not only uses the convention that ENTER creates a return but also has a host of message formatting tools (e.g., font formatting, bullets/numbering, hyperlinks, code snippets).
In short, messaging functionality appears to be pretty extensive but the UI design decision-making is pretty poor.