The key is that they just greatly expand on what they have. And if they attach masteries to defeating raid bosses, they will have effectively added raid progression without alienating players that choose not to participate in it (basically the same as fractals). Think of it more like you have to defeat Teq before you can do Wurm. Or that you need a particular utility skill to defeat Wurm that has no place elsewhere but requires killing Tequatl first to unlock it.
I really don't like gated content, period.
Example: I really want to do the latest LS with my brand new Sylvari character, but have to wait until level 80. I see no reason not to bump up characters for at least the instanced personal story bits (especially if I'm just going to be magically reduced in hit points or turned into Caithe anyway).