1. You lose the fresh content bonus. 2. Your internal link structure changes. 3. The link structure on sites that are linking to you changes. 4. Duplicate [i.e., stolen] content. 5. Your site becomes stale.
Glad this is a sticky... and too bad I had to learn a few of those 5 points the hard way. That's a great guide to helping to stay ranked.
First and foremost, I would completely stop any bad, copied or not spinned properly articles on my blog. Later I will go through all the link exchanges which I have done and check if anyone else has been penalized by google and remove those links. Then would start submission of bookmarkings and directories on various sites. Also start adding new original content and delete all the old articles.