Peter Caputa has written a little piece on what he believes is a flaw in the Blogdex ranking technique:
Peter is completely right, this technique can be used to drive a post to the top of the index (as he has so aptly shown). Since the day Blogdex debuted, people have been devising ways to exploit the ranking algorithm. And truth be told, most of them work :) As of late, a few weblog widgets have been facilitating a regular sort of Blogdex exploitation. Sites that automatically post various pieces of information to the front of their weblog (comments, referrers, etc.) allow for outsiders to manipulate the front of their sites, and in turn, the front page of Blogdex.
Of course the time weighted nature of the index means that while any one person can control the index, they can't control it for long. Every link that gets to the top of the index only stays there for a day at most, and furthermore I'm constantly looking for sites that look out of the ordinary. Even though the type of exploit Peter points out has existed for years, and been taken advantage of a number of times, it hasn't really drawn much attention. I thought about trying to build in some sort of system to detect sites with referral links on their front pages, but truth be told, it hasn't really warranted that much work yet.
The downside of a completely transparent system is its manipulability, but this is of course also what makes it trustworthy. Sans comment spam (which is largely a product of a weblog software exploit, not Blogdex), there hasn't really been a loophole which has continually affected the index. Blogdex is your sandbox... play around and figure it out. Break it and I'll fix it... then I'll study you and get a Ph.D. ;)