How clearly defined the various power sources are is directly dependent upon the willingness of writers, DMs and players to restrict the various power sources. For example, if the Shadow power source was the *only* way to use illusion or necromancy powers, that would be a very clear contrast. However, it would require the Arcane, Divine, Psionic and all other power sources to *not* contain any illusion, necromancy effects. This, of course, is highly unlikely. So as 4e goes along, the designers and home brewers will have to be vigilant in prevent scope creep for each power source, if they desire strong divisions between the capabilities of the power sources.
As far as I can see, the only Power Source whose enthusiasts are keen on defining in any way clearly is the Martial Power source. I don't think the Arcane, Divine or Psionic power sources have fans that will have any significant taste for putting any limits on their scope. There will be some significant, non-cosmetic, differences between these, IMO. However, they will not be particularly strong because no one wants to say what Arcane *can't* do.