core enhancements:

unix threading
on unix systems with posix threads support, apache can now run in a hybrid multiprocess, multithreaded mode. this should improve scalability.

new build system
the build system has been rewritten from scratch to be based on autoconf and libtool. this makes apache's configuration system more similar to that of other packages.

multiprotocol support
apache now has some of the infrastructure in place to support serving multiple protocols. mod_echo has been written as an example.

better support for non-unix platforms
apache 2.0 should be faster and more stable on non-unix platforms such as beos, os/2, and windows. with the introduction of platform-specific multiprocessing modules and the apache portable runtime, these platforms can now avoid using posix-emulation functions that are sometimes buggy and always hurt performance.

new api
the api for modules has changed significantly for 2.0. many of the module-ordering problemsfrom 1.3 should be gone. 2.0 does much of this automatically, and module ordering is now done per-hook to allow more flexibility. also, new calls have been added that should allow
modules to do more without requiring patching of the core apache server.