There is a very large discussion at Sphinn around a Search Engine Journal post named How to Get Rid of Multiple Subpages & Not Get Penalized. The discussion is around the topics of using different techniques when removing or redirecting pages.
Which is better to do for SEO? Do you want to 301, redirect a page, or 404, return a not found status, a page?
I can tell you that my company does this fairly frequently. We take different approaches for different sites. I try, try hard, to not think only about SEO and think what would also benefit the user.
Here is my guide:
301 redirect everything you possibly can, when it makes sense. If you have a page about big blue pineapple chairs on the old site and you are moving it to the new site, 100% use a 301 redirect from the old URL to the new one. However, sometimes it is not that easy.
Sometimes you have a site with hundreds, if not thousands of pages, if not more. Manually redirecting each page is a huge chore. When we can, we set up logic based 301 redirects, to redirect the old URL to the new URL dynamically. That can result in thousands of redirects, but Google should handle those fine over time.
For all the pages that do not match that pattern or logic AND for sites where there is no logic (large, old, non-database driven sites), you want to manually redirect the most important pages. So make sure you have analytics installed on the previous site, way before launching the new site - this way you have the 301 redirects in place, on the most important pages, when you launch.
Then, all other pages, I typically set up a 404 page, returning a valid 404 status code, plus it is set up as a custom 404, so users who land on it, have an avenue to find the right page.
But Rae, at the Sphinn, suggests that sometimes you should think about setting up what is called a "soft" 404 page. Basically, a soft 404 page, is a page that looks like a page not found page, but returns a server status of 200, meaning, the page is valid and active and should not be deleted. The only issue I see with that, is that the URL of the page will be different but the content of the page will be very similar, if not exact, to the other soft 404 pages you set up. Of course, if you are a smart coder, you can look at the Google referrer or the old page's data and serve up contextual relevant product or content on that page, which would make the page's content more unique.
So, like I said, it totally depends on the situation.
I am sure you guys have your own thoughts.
Forum discussion at Sphinn.