You can comment on almost every kind of post on the Landing, if the owner of the post has allowed it. This includes blog posts, wiki pages, photos, files, bookmarks, polls, calendar entries and more. Comments can include pictures and rich text and attached files if you like.

If you have set up 'My account' to send messages (the default) then you will receive alerts each time anyone comments on your post. However, if you wish to follow an ongoing conversation that develops around someone else's blog post or discussion then you will need to subscribe to the conversation.

To subscribe to a conversation, wherever you see a button labelled 'Subscribe to future comments'  like this:

on clicking it you will receive alerts (through the Landing messages and/or email, depending on the settings you have made in 'MyAccount') when people make comments on a particular post that interests you. You may turn these off at any time by clicking the 'unsubscribe' button that appears next to any posts you have subscribed to. 

Note that this is only available on blog posts and discussions created after September 29th, 2010. It is mainly meant for blog comments: it does not yet work for comments on files, bookmarks, pages or photos.