1. No matter how many times you blog it, Digg is not a fair democracy. Bigger sites, that already have more visitors and who post a 'Digg' icon after a post will get far more votes than a site of lesser traffic. Their story also carries more weight and people are more likely to vote it up like sheep anyway. It is a popularity contest.
2. People are being banned left right and centre for pedantic little reasons, e.g.DotmySpot.
3. You get next to no benefit of submitting someone else's article.
4. Traffic is good, but Digg traffic often crashes the site or sends the bandwidth bill through the roof (not on blogger blog as they are free). I am not offering possible solutions, just highlighting the problems.
5. You cannot prevent an article of yours being submitted by someone else. This wouldn't usually matter, but if you wrote an article a year ago which is now out of date, and someone just submitted it, you would look like a damn fool.
6. There are arguably better sites for the same job - I wont mention examples because I don't want to start a war between them...
7. Digg traffic rarely clicks on ads, and so does not equal advertising dollars.
8. Many of the posts are totally off topic of Digg. This wouldn't be so bad but some actually make it to the front page!
9. You can't Digg an article without logging in.
10. "Digg may remove any Content and Digg accounts at any time for any reason (including, but not limited to, upon receipt of claims or allegations from third parties or authorities relating to such Content), or for no reason at all." For no reasons at all!? What's all that about!