Oracle RAC with single node Data Guard physical standby Best practices for Oracle 11gR2 Data Guard implies that the primary and standby databases should have the same layout, eg. RAC to RAC. 

What special considerations would one need to take into account to replicate a RAC database to a single node physical standby?

Any reason why this would be a very bad idea?
There aren't any special considerations that need to be taken into account when setting up a RAC->No RAC Primary->Standby configuration.

In fact, there's a White Paper written by Oracle which explains the setup.

The Oracle documentation is here.