@eskela True, there really isn’t anything “special” from a typographic sense, but from a programmatic sense, some characters are used to denote executable code, command sequences, or un-processed comments, depending on the programming or query language being used.
In common nomenclature these, and the balance of the non-alpha-numeric characters, are considered “special” as they are not normally used in most dictionary words.
While disabling “special” characters is a work around for some sites, until the greater majority of institutions (including, sadly, US-CERT and the FBI’s InfraGard, along with the Department of Defense, Department of the Army, Veterans Administration, etc.) reconfigure their authorization and authentication systems to the most recent NIST /ISO recommendation /guidance, there will still be the need for some “special” characters.
My suggestion is to maybe develop something similar to what Password Safe uses, where there is a “global” and a per-login “Password Policy” setting, where if the per-login policy is not modified, the global parameters apply? It’s already partially in place from what I can see, in regards to password length.