What is a "valid" I-140?
I-140 was "approvable" at the time it was filed.
What does USCIS look for in an approved I-140 if their intent is to revoke it?
"Approvable when filed" normally refers to an I-140, I-130 or labor certification that is filed on or before April 30, 2001. That was the effective date of INA 245(i), a law that provides that valid petitions filed on or before that date will enable the applicant to apply for adjustment of status (i.e., apply for a green card in the U.S.) even though the applicant entered the U.S. illegally, worked without authorization, or overstayed the time permitted under the visa.
Once an I-140 is approved, it can still be revoked if the USCIS concludes that it was approved in error. Before revocation, the USCIS will first send a "Notice of Intent to Deny" (NOID) that summarizes the problems with the case and gives the petitioner a chance to respond. There is a deadline for responding, and it is advisable to hire an immigration lawyer with extensive experience in I-140 issues to assist in preparing the response.