Having a textarea submit on enter breaks with the default behavior. In theory, if you want users to be able to submit a comment by pressing Enter, you should change the textarea to a single line text field. The use of a textarea implies that one should be able to put line breaks in the input.
However, I have to admit I haven't noticed the change. That might be because I've been using the mouse to click the comment button. (Tab + Enter doesn't work on Mac OSX by default -- Tab doesn't focus buttons.) It might be that I habitually press Enter at the end of a paragraph, and a comment is rarely more than a paragraph, so by submitting the form the UI is anticipating what I was going to do next.
It might be the fact that when I do submit a comment prematurely, it's easy to edit or delete. Unlike Twitter, there's no reason to be concerned that a lot of people would have seen the half-baked version or will still have a copy of it after the edit.
Is it good UX or bad? Should others use it on their sites?
I don't know. But here's three things I would take away from this interface:
- Be consistent.
- Test your assumptions, even if the usability tests lead you away from consistency.
- Always provide an easy way to undo.
Edit: I'm now convinced the the SE change was a bad idea. There was nothing wrong with requiring a user to press the button to submit a comment. For no good reason, SE has created a different kind of textarea with no visual affordances to distinguish it from regular textareas. It forces users to learn two different rules for textareas and remember or guess which rule applies where. Don't make me think. :-)