Command + K (Mac) or Control + K (Windows) to open the quick switcher
Move between agenda items
Up and down arrows when looking at the item detail view (remember to unfocus the comment section when you open it up for the first time)
Press "Enter" for a line break within the same text box for summaries and descriptions. Also, "Shift + Enter" will submit information entered when initiated within the summary and description text boxes.
When it comes to comments, pressing "Enter" within an agenda item will submit the comment and "Shift+Enter" will start a new line.