When on the Posts page, I think it would be beneficial to have a “View Post” link that opens that post in a new tab / windows (could make the target an option through the settings).
Often enough I come across the need to view the post without first going to the front-end, search for the post and then view it.
You can already do this by clicking the settings cog for a post and then clicking the view post link unless I’m missing something?
On the “edit post” page, yes - there’s a preview option. On the Posts page in Ghost admin (where you see all your posts), there’s no option to click a link and have the post opened in a new tab.