Here is a list of Google+ keyboard shortcuts that are useful while reading your incoming stream.
Press Shift + ? to get the google+ keyboard shortcuts help window (refer image below)
Details of keys and their actions while on a Google+ stream
j – navigates to next post (use this to skip through posts without using your mouse / scrolling)
k – navigates to previous post
o – expand / collapse current post (this is useful while the post is long and has a expand post link)
n – next comment on the current post (very useful in posts that have lots of comments especially highly recommended or hot posts)
p – previous comment on the current post
r – comment on current post (takes focus to post a new comment in current post)
/ – Focus changes to search textbox in the top
@ – Launches google+ feedback dialog
← – List of pages (pressing left arrow takes focus to navigation ribbon. Use up/down arrow to change highlight. Press ENTER to launch the selected navigation ribbon.)
→ – Page content (pressing right arrow brings focus to whichever navigation ribbon highlighted on the left. for e.g if you have focus on home and press right arrow, focus jumps to your stream)
Can’t remember these shortcuts? No worries, just press Shift + ? anytime and you’ll get google+ shortcuts help window.