When typing the tags for a question, it would be nice if the first tag was selected by default. Most of the time, that's the tag we want but since it's not selected, typing "Enter" posts the question instead of choosing the tag.
I think Enter key should be for submitting the form. Even if you use Enter key for tag selection, hitting the key twice would still submit the form.
This is what I do when submitting questions. It works like a charm, all the time. :-)
Enter your tag phrase. It need not be the complete word, just part of it should do.
You will get a list of tags matching the criteria. Use the down arrow to get to the tags list and press right or left arrow (if you went too far) to navigate the list of tags. Hit space bar to pick the right one.
There it is, the tag of my choice.
I think that is wonderful and simple. If I have to pick the first tag, I have to press down arrow and hit space bar.
Just my thoughts that I like the way it currently works.
You will find that the expected/wanted behavior will diverge amongst users, i for one would absolutely not want that.