SQL to Mongo Mapping Chart
If you prefer to learn by example, the official MongoDB docs have a great table of SQL / MongoDB statement equivalents.
MapReduce and server-side code execution
If you heavily use aggregates in SQL you will quickly miss them in MongoDB. Rick Osborne put together a great infographic on translating SQL to MapReduce (at least in your head). Here’s the PDF version.
Also be sure to check out the reference on server-side code execution.
This gist provides an example (in Java) of executing server-side code when MapReduce is too heavy and in-place updates are not possible.
What are your favorite MongoDB-related resources?