What it probably means is that Dumbledore et al must constantly contend without the authority to take action, especially in light of Fudge, who is probably Slytherinish enough to fire any MoM officials who contend that Volemort^([sic]) has returned.
Source: wiktionary
He was Slytherinish in his early life (which he sort of returns to in Mort, but look at his dedication to Death in Hogfather. People do change (a weakness in the whole sorting system).
Source: wiktionary
He would not have changed; the Slytherinish traits are already there.
Source: wiktionary