Its most notable feature is that it gives the original developer of Mozilla (Netscape, now a subsidary of AOL-Time Warner), the right to distribute modifications made by other contributors under whatever terms it desires, including commercial terms, without granting similar rights to those other contributors in respect of the contribution of the original developer.
This asymmetry in right led to the criticism of the license by many members of the open source and free software movements, most notably Richard Stallman; furthermore, this provision, as well as the patent licensing provisions, make the license GPL-incompatible.
The Mozilla Public License is similar, but lacks the asymmetry in rights. AOL-Time Warner, exercising its rights under the Netscape Public License, and at the request of the Mozilla Organization, has relicensed all code in Mozilla that was under the Netscape Public License (including code by other contributors) to an NPL/GPL/LGPL trilicense, thus effectively removing the GPL-incompatibility.