The only visual indication of whether a patch is in "edit" or "run" mode is via the cursor (toggle with CTRL-e).
Pd does not conform to the right-to-left ordering of output messages. Instead, outputs will happen in the order they were created. It is expected that users will make use of the trigger object to disambiguate message ordering.
Pd has less graphical user interface objects than Max.
Various objects in Max are not in Pd and vice-versa.
In Pd, inlets and outlets in subpatches are named objects, not GUI elements (as in Max).
Pd uses floating-point representations for all computations.