There's a DataObject::$default_sort property which gets applied whenever no sort parameter is explicitly passed. There's currently no way to specifiy this directly for ModelAdmin, but its on the cards for a more flexible tabular representation in the future.
You can use ModelAdmin::resultsTableClassName to customize the TableListField class being used for results. You can then create a subclass of it to provide custom functionality. If you just want different sorting, you can modify ModelAdmin->ResultsForm(), YourDataObject->getSearchContext(), or even just the default ordering through DataObject::$default_sort :)