Trend: Comment Submit Button Bye Bye
A trend not to provide explicit submit buttons in the GUI for committing comments on social websites can be identified. Instead the user submits by pressing the Enter key on the keyboard.
Two site examples
Facebook, for one, has recently removed the submit button, as is evident from this screenshot (the submit button used to be in the lower right corner):

As another example, the Q&A forum for user interface professionals UI.Stackexchange.com has a somewhat confusing combination of an explicit submit button (“Add Comment”) AND a submit-on-Enter action:

Possible implications of removing submit buttons
So, what are the possible implications of the removal of explicit submit buttons in the GUI/the introduction of submit-on-Enter actions?
One possible (and most likely) implication is the premature submitting of comments. As the user unfamiliar with this mode of interaction is hitting Enter to insert a line break/carriage return s/he instead publishes her/his unfinished comment.
This is of course only a concern until the user becomes familiar with the feature. However, re-mapping the primary action of a (very) familiar control may frustrate users. And although Facebook has never been about advanced/heavy formatting, the (apparent) lack of such a basic formatting feature as line break/carriage return, may spur annoyment as well.
To circumvent the situation users may post either longer unedited strings of text or shorter, more fragmented comments.
OR they must become familiar with the hidden shortcut such as holding down Shift while pressing Enter to insert a line break/carriage return instead of submitting the comment.
The “X” still means “Remove” (“Fjern” in Danish)…:

…but has a special feature for a short while after comment submission: To initiate an edit mode where the shortcut to inserting a line break/carriage return is promoted below the text field:

On a side note, if the user wants to delete the comment while in edit mode, all text must be deleted and the Enter key pressed.
Summing up
To sum up, it’s hard to tell whether this is an improvement in terms of usability and user experience. It will most probably frustrate a lot of users as is evident from an opportunity to get back the explicit submit button: http://sumtips.com/2011/03/get-back-old-facebook-comment-button.html
On the other hand, it reduces the amount of perceived GUI “bloat” and perhaps this new approach will encourage people to post just one point per comment (a more positive perspective on the fragmented comments issue) — or to post less characters per comment (which may benefit speed of reading – and of course the storage requirements on the part of the web site owner).