--amendoption. Git has had this for a while. Before 2.2 you could get the same functionality in Mercurial by using the mq extension, but it took at least 3 commands (qimport, qrefresh, qfinish). It's nice that you can do it with just one command now. Mercurial's relatively new phases come into play with
--amendin that, by default, they will prevent you from amending a commit that has been pushed to or pulled from a remote repository. It's a nice little safety net to have, and of course you can override that behavior if you need.
It's nice to see this incredibly capable and easy-to-use tool get even better.