Currently SO employs the reputation score for every user. After you have familiarized yourself with SO, you come to know that Jon Skeet has highest reputation score and Marc Gravell is after him. It is natural that you will favor the answer of a user with highest reputation which in other words means he has good knowledge and reliability.
I bet there are huge number of users who don't even know who these two guys are or who possess the highest reputation score/number. Or consider if a user puts his reputation badge on a CV or an employer visits the SO's career site, the mere number does not really help an employer decide what is the overall ranking of this person he is viewing the CV of. Same goes true if I put the reputation badge on my blog. When asking a question, sometimes less reliable/fragile answers are favored over the better answers by people who are authority in a particular topic.
For this reason, I have always felt the need of overall ranking system. Any SO user, employer, your blog's visitor can see what is your overall standing at SO or in other words, how good you are at your skills (excluding some great guys having less/no reputation, Matt Mullenweg for example). Anyone could easily see that yes Jon Skeet is the guy I need to trust/hire. A big big boost by the SO to a programmer!
Here is what I have always felt the need of:
Please share your ideas.