What are the new features in R12 in a nutshell?

Set of books are now called ledgers.A central workbench called Accounting Setup Manager is available in R12.
R12 allows you to map LE to an entire ledger or balancing segment values within a ledger.
Set of books in 11i was defined by 3 Cs,in R12,the ledger is defined by additional C;the sub-ledger accounting convention.In 11i sub-ledger accounting convention was defined at OU level.To access more than one ledger using a single responsibility given that all those ledgers share the same Calendar and Chart of account .(MRC) and journal translations have been placed under broader concept called reporting currency.The new feature of secondary ledger.New security feature called'Access Set'and there is no need to install (ADI) as a desktop client software any more are some of the new features of R12.

Oracle Payables:
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In Oracle 11i Payables, invoice Approval engine incorporates the invoice validation process. Whereas, in R12, the invoice Approval program and the invoice Validation process are two separate things.

Very high level: (Interview Answer)
Multi-org access (MOAC)
Subledger Accounting
Ledger Sets
Accounting Setup Manager (Secondary Ledgers, Reporting Ledgers)
Definition Access set
Data Access Set
eBTax
Global Intercompany System
Centeralized Banking
Reporting including XML Publisher
Oracle Payments