The WordPress default theme (like 2010) shows the full content on its front page. Even themes released by Automattic is designed in such a manner.
There is no option in the WordPress backend whereby it can be defined that the front page should contain excerpt.
Moreover, with themes getting more complex - with different models of php structure being used, its really difficult to find as to how to make the excerpt show on the front page.
(WordPress themes - even those released by automattic - are becoming phppress)
It would be really nice - if there is an option in the WordPress backend options eg. in "READING" options where one can define whether one wants full posts or the excerpt on the front page.