For now, if you have default parameters for a Puppet class, you have to check "override" in Puppet class configuration for a parameter to be able to override it.

Most of modules I use (and I do nothing exotic) have defaults ($params), and in my environment, I often need to change the defaults for some hosts or groups.

To make things easy, a global parameter "override_all" would be welcome. Set it to false by default and you won't break anything existing. When set to true, you don't need to activate the feature for every parameter, you have direct access to the override section for hosts, groups and config groups.
If no value is provided, you just use defaults, else, you put whatever you want for your hosts/groups.

I would really like to have this to have a complete "Hiera like" behavior, but with the powerfull features of Foreman.