You can tell how many total comments a story has by the color of its title. The ranking from low to high is: dark blue -> light blue -> aqua -> green -> yellow -> orange -> red.
It has a keyboard-centric interface. Here is the breakdown:
'Space' toggles an item's open/closed state.
'Tab' is used to move between items in a list. If an item is in the open state, pressing tab will focus the first item in its list. Otherwise, it will focus the next item in the list that the item is a member of. This is really the "go to" key in the app. If ever in doubt, just hit 'tab'.
'Enter' is for opening the link (in a new window) to which a "story" item refers. (You can also click the link to open it.) When a notice (see below) is in focus, hitting 'enter' should bring you directly to the item that it refers to, in an opened and synced state. Doing this might still fail, though, which is why there is a "hard reset" option in the app.
'c' is for popping open a comment window to respond to whichever item is in focus.
'n' is for setting a "notice" to periodically poll the backend for new replies to a given item (story or comment). The current polling period is one minute.
'/' is for cycling between the 3 screens of the app (Top stories, Watched stories and Notifications).
'1' is to jump to the "Top stories" screen. (All story titles that start out as "Launch HN:" are filtered out of the listing.)
'2' is to jump to the "Watched stories" screen. "Watched stories" is the screen that will hold the top level story items that contain notification items that are no longer embedded in the DOM of the "Top stories" screen.
'3' is to jump to the "Notifications" screen.
'x' is to delete a given notice. The app currently allows for 5 notices to be set at any given time.
'r' is used to refresh a given item to sync it with the firebase backend.
'Esc' allows you to "escape out" of embedded items. This key also automatically closes items.
'Ctrl+Esc' allows you to immediately escape out to the top level listing, closing all embedded comments along the way.
Finally, since there may still be a bug or two in the app's logic, 'Alt+l' is the "hard reset" shortcut that forcefully closes and blurs every item on every screen. If you need to invoke this shortcut, just pressing 'Tab' again should get you back on your feet.
The link I provided above allows you to open up a Linux on the Web app in the experimental "app mode". If you want to play around with it in its native environment, though, you will need to need to see the LOTW README: https://github.com/linuxontheweb/LOTW#applications