Interesting posts scarecrow, thanks.
I'm not sure if this fits with what you're suggesting, I changed domain names 2.5 months ago on a well established site, growing fast, decided to switch before it got bigger and suffer now rather than later.
301'ed old site to new site, all old content deep in sandbox re SERPs, however most new pages and content I'm adding are not in sandbox, but rank fine for their specialized terms, about the same as anything I put in before I switched domain names. Two possiblities that I can see:
If new content is not sandboxed and old content is, google is comparing the old site to the new site somehow, and letting in the new stuff but keeping out the old.
it's just the original pages that are sandboxed? But that doesn't fit with what many people are reporting, full sitewide sandbox on a new domain name, all pages.
But it's hard to see how that would happen on only old pages and not new ones without some reference to the site as a whole if a sitewide sandbox exists, which it doesn't seem to in my case, in other words, google is differentiating between the group of old urls and the new urls despite the alleged sitewide sandbox affect. Many of the 'old' urls have been extensively rewritten during this time, but still are buried, so it's not the content per se of the pages.
<added>another oddity: after 2.5 months, doing an site:olddomain.com which has been gone, 301, now for that whole time, reveals 3 pages, very old, 2 haven't been physically present on the site for about 10 months, from a section of the site I took down, but which is still linked to I guess.