This means first that everyone needs to authenticate via OAuth before using any kind of API request. All the non-authenticated API usage, which was sometimes nice for quick implementations will be gone.
In version 1.1, we will require every request to the API to be authenticated. [...] If your application is currently using the Twitter API without using OAuth, you will need to update your application before March 2013.
Then the limits. They have always been a pain for some usage. They will be restricted in some way.
In version 1.1, we will provide per-endpoint rate limiting on the API. While an application that only accesses one endpoint may be more restricted, applications that use multiple endpoints will run into rate limiting issues less frequently.
Most individual API endpoints will be rate limited at 60 calls per hour per-endpoint. [...] high-volume endpoints related to Tweet display, profile display, user lookup and user search where applications will be able to make up to 720 calls per hour per endpoint.
Based on analysis of current use of our API, this rate limit will be well above the needs of most applications built against the Twitter API, while protecting our systems from abusive applications.
Hmm. 60 calls per hour sounds not so much for me for posting tweets. But we will need to wait and see which API will be limited.
Then there will be lots of changes to the guidelines for developing against the Twitter API. These changes worry some people, which include Twitter in products currently.
[...] Display Requirements, which we will also introduce for mobile applications. We will require all applications that display Tweets to adhere to these. Among them: linking @usernames to the appropriate Twitter profile, displaying appropriate Tweet actions (e.g. Retweet, reply and favorite) and scaling display of Tweets appropriately based on the device. If your application displays Tweets to users, and it doesn’t adhere to our Display Requirements, we reserve the right to revoke your application key.
Lot’s of changes which can be read in full length at dev.twitter.com and which will lead to lots of LOC changes.