There are two kinds of packages that are not likely to be accepted as new contributions to PEAR:
A package that duplicates an existing package very closely
A framework or other full-fledged web application
There are exceptions to the duplication rule, for instance a package that requires a higher PHP version, and takes advantage of newer versions in PHP will definitely be considered seriously. Also, packages that provide a completely new paradigm for accessing the same thing are a good candidate for consideration.
Frameworks or other applications may also be considered if they are developer tools, such as phpDocumentor. However, if you have a great idea that you think would fit into PEAR, don't let this document discourage you. PEAR developers can be surprisingly open to new ideas. Remember, the best ideas are well documented and carefully presented. Be prepared to have patience and thick skin! We are a picky bunch about quality, but in the end we will all learn a lot and develop some stellar code in the process.
As the parameters for proposing a package change, this site will be regularly updated.