These days you can't:
- use blog comments
- use articles
- use press releases
- use guest posting
- use web 2.0 blogs (apparently mentioned at SMX Advanced)
- use paid reviews/mentions (nofollow "forced" by Google)
This is something I've never really gotten - I find that most quality sites DO indeed have a large number of outbound links, linking to other relevant resources, more information, related blogs, and so on. News sites often do this, wikipedia has the entire bottom stacked with outbound links, and if you go to a decent university you're told to look for sites that can verify their sources (often with outbound links).
So... what gives?