The five mistakes (expanded on in much better detail in the blog post itself) are:
Using MySpace as your website
Using MySpace as your email
Having an impressive background image
Embedding lots of media
Writing lots of text
All of these points are absolutely correct. MySpace’s “My Pics” should not be your gallery. By all means, put loads of examples of your work in there (if they comply with MySpace’s image content rules) but don’t direct people there instead of to your own site.
Andrew Dubber reminds us:
Remember: MySpace is a tool. It’s one of many. It’s not your only shot at engaging with your audience or prospective market. It’s an important one though, and it’s one that it’s very easy to make mistakes with. Use it well.
MySpace, like any social networking site, should just be one of many outposts of your main website. In no way should it be your website (and nor should Flickr, deviantART, Artbreak, or any other social website).