Bernie Sanders has led his campaign with issues not mudslinging. In facing people with different ideologies, Bernie Sanders has looked them in the face and said that he disagreed with them. He then asked them to put that aside so they all could work together on more important issues. Issues are his map to the White house.
Ralph Nader was pretty progressive too, but he wasn't quite as outspoken about changing things quite so much. Sanders has some truly progressive ideas, and is not afraid to speak out loudly in support of them, or in condemnation of ideas that are entrenched into society today. He is very progressive.
A lot of presidential candidates try to appeal to moderates by keeping their opinions close to the political middle. Bernie Sanders is one of the few candidates in recent history to take a strong side with the left. While many don't agree with his self-proclaimed socialist views, there are a lot of people that do side with him because of his passion.
No, Bernie Sanders is not more progressive than all other recent presidential candidates. Hilary Clinton is more progressive in many respects. For example, her unapologetic support for abortion rights is very progressive. She has also demonstrated consideratble support for other progressive agendas, such as the right for gay couples to marry.