We're at the point now in 4E's development that we can see some of the holes in the power source idea, and it highlights part of the problem of trying to jerryrig the system to fit classes into it.
I think we can all agree that Martial makes sense as power source. The energy of the classes comes from within the body. You can argue whether Ranger should be in it or not (and if it was to get removed, you'd basically just replace it with the 'Archer' class)... but the concept and application of the source works. Personally... I do think something should be added to the source to really tie it together in the next iteration of the game (whether that be the Essentials idea of 'basic attacks with mods' or else making weapon groups more prolific over all the Martial classes and not just Fighter.)
Psionic power coming from the Far Realm? Works great. Monks get removed from it obviously, but otherwise, it's a great idea and you can keep the whole concept of Augments to retain some mechanical flavor to the classes.
The Divine source also works for what it's supposed to... classes which are granted their power from divine beings, or more to the point, taken directly from the Astral Sea.
The problem here though... is that WotC didn't follow through with this way of thinking. By rights, the Elemental source should therefore be power gifted by the creatures of the Elemental Chaos or from the Chaos itself (demons, elemental lords and the like). The Shadow power source is gifted or powered by the Shadowfell.
And here's where the biggest screw up is for their power source concept... Arcane should be directly powered by the Feywild. The Feywild as spec'd has an overabundance of arcane energy and pretty much all the creatures there use it and are infused by it. But the problem is... because the Feywild is considered by pretty much everybody to be more about NATURE, rather than MAGIC... you have the conundrum of creating Arcane classes that have nothing to do with the Feywild, and instead Arcane becomes this nebulous source where dragons power sorcerers, music powers bards, putting elementals inside objects powers artificers, and warlocks negotiate with archfey, far realm creatures, devils, and their own ancestors to get their abilities! What the hell? The truth is... the Arcane source is actually NOT A POWER SOURCE AT ALL. It's a label that gets assigned to any class that uses the nebulous term 'magic'... regardless of where that 'magic' comes from.
And the stupid part of all this is that the one power source which is ENTIRELY ABOUT NATURE (Primal) and should be all over what the Feywild is about... is completely fluffed AWAY from the Feywild. And WotC had to create this whole concept of 'primal spirits' to grant the primal classes their powers.
Truth be told... the Arcane and Primal power sources should really have been swapped. Primal should be all about getting their power from the Feywild and the creatures and nature within it... and Arcane should be all about the energy that the primary world (and its denizens) just have. Whether that be the sorcerers who can siphon arcane energy off the the dragons in the world... the wizards, bards, and swordmages who can tap into the ley lines of arcane energy and channel them through implements, instruments, and weapons... or the runemasters, alchemists, and artificers who can siphon ley energy and store it into objects to use at a later point.
And warlocks? They're the special ones who get to go cross-source, as they can be Primal for the Fey Pacts, Divine for the Infernal Pacts, Psionic for the Star Pacts, Shadow for Gloom Pacts, and Arcane for the Vestige Pacts.
You do this kind of thing... making the power sources ACTUAL and CONSISTENT fonts of energy from which PCs get their abilities... and you can start to have them retain a little more meaning that they have in the past.