In Maple 11, there were two separate engines shipped with Maple, one single threaded and one multi-threaded.  For Maple 12, these two engines have been merged into one.  Now the kernel has two modes, a single threaded mode and a multi-threaded mode.  The single threaded mode is enabled by default.  When a function from the Threads package is executed, the kernel enters the multi-threaded mode.  It remains in multi-threaded mode until the next restart.  For more information on the multi-threaded engine see the multi-threaded help page.

The stop conditions facility available for numerical ODE solutions in Maple 11 and prior has been replaced by a new mechanism for handling more general events. See Events for dsolve[numeric] for more information.

Plots generated by the plots[polarplot] command are now displayed with polar axes by default in the Standard Interface. It is possible to get the Cartesian axes produced in earlier versions of Maple by using the axiscoordinates=cartesian option.