This can indeed be done by simply mentioning a users name, either by typing it or by typing a + or @ followed by their name or e-mail address. However this will not work for an existing comment, either when editing or when replying. It only works when creating a new comment.
One used to be able to assign a task just by typing someone's name.
That way has changed.
You will now be given the option to assign a Task (Action Item) as soon as and only if you type either the @ or + characters.
Following that you can choose who to assign the task to.
You can use the same technique in Google Spreadsheets as ...
Trello does not currently have a sort cards function.
You can put those values in the card description.
Manual sort by dragging and dropping cards may be your only option right now. Easier than changing values for some of those subjective points.
I figured out how to do this. Look for "My calendars" on the left. Click it to expand it. You should see a row/item labelled "Tasks". If the checkbox next to "Tasks" is not already checked, check it. Now you should see a sidebar on the right appear, labelled "Tasks". This is the list of tasks. Click any empty area in that sidebar, and that will let ...