The conservative garbage collector (CGC) has been replaced with a precise garbage collector (3m) in the standard build. For most users, this change simply amounts to "better performance in space and time". For example, a long-running DrScheme instance typically uses much less memory than before.
The new memory manager also supports a new "Limit Memory..." option (in DrScheme's "Scheme" menu) to limit the memory use of a programming running inside DrScheme.
For those who work with C-implemented libraries and extensions, the switch to precise collection may complicate interoperability. To a large extent, however, (lib "foreign.ss") works the same with both collectors. (But note that the 3m is a moving collector, so be careful with passing Scheme objects to C.)
Although our pre-built binaries use the new collector, builds from source using the conservative collector are still supported.
For a program written with one of the the "How to Design Programs" (HtDP) languages, DrScheme saves the program with meta-information that identifies the language and records the teachpacks used by the program. DrScheme's teachpack GUI now works only with the HtDP languages. In other languages, use require to access teachpacks.
The meta-information is in the form of a reader extension that turns the file content into a module-based program, which means that teaching-language files can be loaded directly into MzScheme or used with other PLT Scheme tools.
The HtDP "world.ss" and "image.ss" teachpacks have been revised, including support for the creation of animated GIFs.
Unit-based servlets are no longer supported in the web server. Use module-based servlets, instead. (Servlets can be implemented using a unit within a module, but the web server's API is provided through a module.)
A new (lib "unit.ss") library replaces the old one and provides a simpler and more flexible syntax. The (lib "unitsig.ss") library is deprecated but still available as (lib "unitsig200.ss"), and the old (lib "unit.ss") is available as (lib "unit200.ss").
The PLT Scheme Team