Mercurial will attempt to push all branches. It will then immediately stop with an error message unless you specify -f or --new-branch if you're trying to push a branch that does not already exist in the remote repository (or even to create a new head in an existing branch). In order to get rid of the error message you need to either (1) explicitly tell Mercurial to push all branches, (2) specify which existing branch(es)/revision(s) you want to push (they must already exist remotely), or (3) hide the branch with hg phase secret -f. This means that your local branches will not leak into the remote repository unless you specifically instruct Mercurial to do so.

Of these, only the phase approach is new (2.1+). The rest hasn't changed.