http://httpd.apache.org/docs-2.0/new_features_2_0.html.en
Core Enhancements:
Unix Threading
On Unix systems with POSIX threads support, Apache can now run in a hybrid multiprocess, multithreaded mode. This improves scalability for many, but not all configurations.
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 is faster and more stable on non-Unix platforms such as BeOS, OS/2, and Windows. With the introduction of platform-specific multi-processing modules (MPMs) and the Apache Portable Runtime (APR), these platforms are now implemented in their native API, avoiding the often buggy and poorly performing POSIX-emulation layers.
New Apache API
The API for modules has changed significantly for 2.0. Many of the module-ordering/-priority problems from 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 provide additional module capabilities without patching the core Apache server.
IPv6 Support
On systems where IPv6 is supported by the underlying Apache Portable Runtime library, Apache gets IPv6 listening sockets by default. Additionally, the Listen, NameVirtualHost, and <VirtualHost> directives support IPv6 numeric address strings (e.g., "Listen [fe80::1]:8080").
Filtering
Apache modules may now be written as filters which act on the stream of content as it is delivered to or from the server. This allows, for example, the output of CGI scripts to be parsed for Server Side Include directives using the INCLUDES filter in mod_include.
Multilanguage Error Responses
Error response messages to the browser are now provided in several languages, using SSI documents. They may be customized by the administrator to achieve a consistent look and feel.
Simplified configuration
Many confusing directives have been simplified. The often confusing Port and BindAddress directives are gone; only the Listen directive is used for IP address binding; the ServerName directive specifies the server name and port number only for redirection and vhost recognition.
Native Windows NT Unicode Support
Apache 2.0 on Windows NT now uses utf-8 for all filename encodings. These directly translate to the underlying Unicode file system, providing multilanguage support for all Windows NT-based installations, including Windows 2000 and Windows XP. This support does not extend to Windows 95, 98 or ME, which continue to use the machine's local codepage for filesystem access.
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Module Enhancements:
mod_ssl
New module in Apache 2.0. This module is an interface to the SSL/TLS encryption protocols provided by OpenSSL.
mod_dav
New module in Apache 2.0. This module implements the HTTP Distributed Authoring and Versioning (DAV) specification for posting and maintaining web content.
mod_auth_digest
Includes additional support for session caching across processes using shared memory.
mod_charset_lite
New module in Apache 2.0. This experimental module allows for character set translation or recoding.
mod_file_cache
New module in Apache 2.0. This module includes the functionality of mod_mmap_static in Apache 1.3, plus adds further caching abilities.
mod_headers
This module is much more flexible in Apache 2.0. It can now modify request headers used by mod_proxy, and it can conditionally set response headers.
mod_proxy
The proxy module has been completely rewritten to take advantage of the new filter infrastructure and to implement a more reliable, HTTP/1.1 compliant proxy. In addition, new <Proxy> configuration sections provide more readable (and internally faster) control of proxied sites; overloaded <Directory "proxy:..."> configuration are not supported. The module is now divided into specific protocol support modules including proxy_connect, proxy_ftp and proxy_http.
mod_negotiation
A new ForceLanguagePriority directive can be used to assure that the client receives a single document in all cases, rather than NOT ACCEPTABLE or MULTIPLE CHOICES responses. In addition, the negotiation and MultiViews algorithms have been cleaned up to provide more consistent results and a new form of type map that can include document content is provided.
mod_autoindex
Autoindex'ed directory listings can now be configured to use HTML tables for cleaner formatting, and allow finer-grained control of sorting, including version-sorting, and wildcard filtering of the directory listing.
mod_include
New directives allow the default start and end tags for SSI elements to be changed and allow for error and time format configuration to take place in the main configuration file rather than in the SSI document. Results from regular expression parsing and grouping (now based on Perl's regular expression syntax) can be retrieved using mod_include's variables $0 .. $9.
mod_auth_dbm
Now supports multiple types of DBM-like databases using the AuthDBMType directive.
mod_auth_db
Has been removed in favor of mod_auth_dbm with the AuthDBMType directive.