It would be handy if we documented any and all system limits in a single place. Examples:

• maximum document size
• maximum number of indexes per collection
• maximum namespace length
• maximum number of fields per index
• maximum number of members in a replica set (voting and non-voting)

probably there are lots more around.

Additionally, certain quantities don't have limits per se, but are a function of something else about the installation, so it'd be useful to give approximate formulas for an installation's limit on things. Examples:

• maximum number of namespaces (a function of the .ns file size)
• maximum dataSize (an approximate function of the machine architecture and whether journaling is enabled)
• maximum number of chunks per sharded collection (this is mostly hypothetical)
• maximum sharded collection size in bytes (another hypothetical quantity)