Change Log

All methods are now available under the /2.1/ path, new properties will only be returned if the /2.1/ version of a method is called.
Limited support for writing via the API, restricted to comments.
/users/{ids}/top-answer-tags and users/{ids}/top-question-tags are now vectorized.
write_access and private_info scopes added to OAuth flows.

All API request now go to api.stackexchange.com with a site parameter, rather than per-site endpoints as in V1.x.
All methods found on Stack Auth in V1.x are now found under api.stackexchange.com, but do not require a site parameter.
All method now return a common response wrapper object, which places data in the items property.
Throttle and request quota information is now returned as part of the reponse body, not in headers as in V1.x.
DEFLATE is available as a response compression option, joining GZIP from V1.x.
callback is now available for JSONP in addition to jsonp from V1.x.
All returned fields are now safe to inline in HTML by default, further discussion under the safety topic.

/badges/{ids} now returns badges.
/badges/{ids}/recipients replaces the old /badges/{ids}
/questions/{ids}/comments and /answers/{ids}/comments no longer work for the incorrect post type.
/revisions/{ids} now takes revision.revision_guid instead of post ids, /posts/{ids}/revisions replaces the V1.x method.
/users now returns unregistered users in addition to registered users and moderators.
/info has replaced the /stats method from V1.x.
The featured sort is no longer available on /questions, the new /questions/featured method should be used instead.
/users/{id}/associated now expects account ids to be passed, not association guids. Assocation guids are deprecated.

It is now possible to authenticate Stack Exchange users via OAuth 2.0.
Filters make it possible to optimize both the response size and request time of queries to the API.
In conjunction with authentication, a series of per-user throttles are now available in addtion to the per-IP ones from V1.x.
Every method of the form /users/{ids}* now has a /me version, which is the same as calling the /users/{ids} version with the user id of the passed access_token passed.

Version 2.1