"RavenDB supports multi document (and multi node) transactions, but even so, it isn’t recommended for common use, because of the potential for issues when using distributed transactions."
So, the differentiator is that FoundationDB is built from the ground up to support these type of transactions at high performance levels with no "potential issues".
That remark seems a bit unfair. That document is two years old, from long before version 1.0. RavenDB is now at version 2.0. It was also built with transaction support from the start. The default setting for TransactionMode is Safe (http://ravendb.net/docs/server/administration/configuration) so most users will use that, if there were any issues with it that would certainly be known by now.
The current RavenDB documentation still warns against using "system transactions" because of performance reasons (http://ravendb.net/docs/client-api/advanced/transaction-supp...), so I think it is still fair to say that RavenDB was not designed for applications requiring high performance cross node transactions.
System.Transactions refers to support by RavenDB of the Distributed Transaction Coordinator service on Windows. This makes it possible to enlist transactions in multiple systems (MSSQL, MSMQ, RavenDB, NServiceBus) in one single transaction with the possibility of a rollback. It is not needed for transactions inside RavenDB, and I think foundationdb does not even support something like that (it is Windows specific, although Mono supports it on other platforms).
"RavenDB supports multi document (and multi node) transactions, but even so, it isn’t recommended for common use, because of the potential for issues when using distributed transactions."
So, the differentiator is that FoundationDB is built from the ground up to support these type of transactions at high performance levels with no "potential issues".