no submit button for commenting

Maybe a good compromise would be to combine the two, and have a button control with the (+) icon and styled text on it. I'm not sure how easy this is to get across browsers, though.

People spend more time on other Web sites than they do on yours. On practically every other Web site, the control for submitting text that you've entered is a button, usually without an icon. There's nothing obviously wrong or confusing about this pattern. Therefore, the control for submitting a comment on Launchpad should be a button, and it doesn't need an icon.

There's an interesting confusion in Launchpad about what a link with a "(+)" button does: (1) add something immediately (e.g. "Subscribe") or (2) request further input and then perform an action (e.g. "Subscribe someone else"). That confusion should be fixed separately, but adding a third meaning -- (3) submit information that you've already entered -- is a backward step.

Agreed, although I wonder how we can let users know we're not sending them to another page, as they're used to. Maybe it's not so important in this case.
Bjorn, could you tackle this?

We could adopt a new convention, similar to the one where in-page links are green. Maybe draw some kind of border around the button, or add some standard character to the button text.

In this case, I don't think we have to distinguish that the submit button performs an AJAX action. The green link to indicate AJAX is nice where the AJAX action is unexpected; for example, opening an overlay or subscribing inline. In both those cases, the user doesn't expect those actions from clicking a link so the green may be a nice hint (although you still have to be a user fairly familiar with Launchpad to get this convention).

In the case of a "Comment" textarea and a submit button, the fact that the comment is added inline seems inconsequential to the action of submitting. By clicking a submit button, the user understands the comment will be submitted. The page can then be updated inline or could be reloaded to show the new comment and neither is a surprise to the user. So I don't think a standard submit button with the comment then added inline is a problem here.

Twitter, Facebook, and Flickr -- the first three examples I can think of -- don't seem to use any special styling to show that a form submission keeps you on the same page. And however much you need to know that, you don't really need to know it until after you've clicked the button, so I think a spinner after you've clicked the button is quite enough (as long as the spinner is relatively close to the button).

Yeah, I think I'm convinced.

Reported the "Subscribe" link inconsistency as bug 409376.

Fixed in devel r9072.