That said, there are places where leading the party means that you can take all the decisions without consulting anyone. North Korea for example.
And those places are called dictatorships. Presidents can always be blocked by congress. They're not powerless but they don't have unlimited power either which is usually a good thing (but most of the time the president and congress have the same agenda so they can team up to do whatever they want (but they don't seem to like Obama much).