I had no idea what he was talking about until the replies. You and grandparent are wrong. Standard behavior on web forms is tab moves you to the next element.
You are expecting gmail to act like a desktop app, and maybe you could make the argument that it should, but that's a different issue.
We can definitely make the case that it used to and that it doesn't anymore. Sending an email isn't like filling in several text boxes on a web form, it's composing/editing a lengthy message that often has proper formatting like indentation.
Ctrl-Enter is the one keyboard shortcut I wish I could disable. It is my most common mis-hit, usually when pasting something into an email and then hitting enter to advance to the next line. I also miss the ability to tab within the compose box, but on the web I pretty much always assume Tab is not going to indent text I'm typing.
You are expecting gmail to act like a desktop app, and maybe you could make the argument that it should, but that's a different issue.