1. Short pages created by letting location + category be indexed... or any category be indexed that had few results.
2. Multiple versions of a page.
3. same product descriptions on multiple products with say just color different.
4. Dupe content with other sites, regardless of who was the originator.
5. silly pages being indexed. 1 site had 7 pages per listing being indexed, one for contact form, one for gallery, on for send to a friend etc, all thin.
6. Wrong canonical tags = canonical for www.sitename.com/ = /home so effectively say the home page, was not the right page.
all sorts of things.