What are the limitations of a standard mechanical license?

A standard mechanical license is generally limited to the compulsory license terms.  A mechanical license does not permit the use of an existing recording, whether by sampling or other reproduction.  For example, a mechanical license for Michael Jackson’s “Billie Jean” only permits a new original recording of that song (e.g., a recording of a new performance of the song using your own instruments, vocals, etc.)—it does not permit the use of any portion of Michael Jackson’s actual recording of “Billie Jean.”  A separate master use license is required to use the recording, which generally must be secured from the applicable record label.  A mechanical license also generally does not permit your new recording of the song to be used with video, including music videos, films, commercials, or otherwise.  A “synchronization” license is required to use a song with video, which in general must be secured directly from the publisher/songwriter.  There is no fixed rate for master use licenses and synchronization licenses.  These licenses must be negotiated with the applicable rights holders and license fees can vary widely.  Also, standard mechanical licenses may not cover the exact duplication of the order, arrangement, and selections of songs on a third-party album to the extent the third party may have a compilation copyright in such order, arrangement, and selections.  So, for example, a standard mechanical license may not permit you to create your own version of Michael Jackson’s “Thriller” album if you intend to cover all the same songs and compile them in the same way as on the original “Thriller” album.