The keyboard shortcuts that I find the most useful in Google Reader are:
j/k – To move up or down in items. This automatically opens each item in list view.
n/p – Scan up or down. This allows you to scroll through titles in list view without opening items.
s/shift+s – Toggle starred status of an item.
u – Toggles full view mode, so you can scan through items without the left side navigation menu cramping your style.
gh – Go Home.
ga – Go to All Items.
m – Mark an item as read or unread.
Shift+a – Mark all items as read.