What Makes a #1 Defenseman a #1 Defenseman?

One of the larger questions I've grown to ponder over the last week when putting all of these posts together about Paul Martin  (recap: here, here, here, here, here, ultimately concluding that he's the most effective defenseman available on the marker this summer and concluding that the Devils should re-sign him) was what makes a #1 defenseman a #1 defenseman?  Prior, I had the notion that the ideal defenseman would be an impact player at both ends of the rink, able to put up a significant amount of points whilst providing solid defense. Someone who can command a game from the back and generally be in the right position all the time.

However, the more I think about it, the more I think that such a defenseman may be so exceptional and so rare that the meaning is obscured somewhat?  Isn't a #1 defenseman someone who can lead a blueline, regardless of flaws? Given that the most important role for a defender is to actually, you know, defend, wouldn't such a definition of a #1 defenseman shut out all the top defensive defensemen in the league? 

In any case, I'm interested in two questions:

I know on-ice impact isn't the be-all, end-all of measuring a defenseman's skill.  There are other stats out there that help determine it, but I would think that a defenseman who plays against tough competition and generally ranks among the best at even strength play in terms of on-ice impact, especially, in terms of reducing shots against says plenty about a defenseman's effectiveness.

Incidentally, here's how Paul Martin ranked in on-ice impact (and adjusted Corsi/60) in 2008-09 and 2009-10 among all NHL defensemen who played 20 games and had a time on ice per 60 of at least 15.  I wish I did that in Part 1 and Part 2 from last week, but here it is now for your table-reading enjoyment. Sorted by the number of ranks in the top 30 in each season, followed by quality of competition. Only Nicklas Lidstrom and Duncan Keith (italicized in both tables) factored better than Martin in both seasons.   So I'm confident in saying that Martin has an on-ice impact like a #1 defenseman.  What more does he need to do?

Let me know your answers to each in the comments.  Remember, I'm most interested in criteria that determines who really is and is not a #1 defenseman as well as whether Paul Martin meets it or not.