So... I had a random idea for both an explanation and some expansions to Magic Resistance/Barbarian Magic Resistance. The following requires a basic understanding of current magic theory.

The premise:
All living and/or thinking beings have an innate ability to touch the mana streams that run throughout the world. To the untrained (i.e. non-mages), this analagous to walking through a dense forest, brushing the trees aside, but making no real lasting change. This is the basis of basic Magic Resistance, because mana streams are more difficult to manipulate as a result of these brushes. Because mages are capable of "seeing" mana streams (at least their own type) they can pass through the metaphorical forest without hitting the branches, which results in neither innate resistance for them, nor in interfering with their own magics. Since all undead that I am aware of are either concious or the product of magic, either their influence on the streams, or their animating magic interacting with the streams results in their innate resistance, with the same applying for constructs.

Barbarian Magic Resistance:
Barbarians, have been trained in the opposite direction from mages, focusing their efforts on causing more impact. The forest analogy would be a madman running through the forest, pinwheeling his arms and yanking on tree branches. Thus, any magic done in their area would have to be formed from mana streams that are being pulled on and rattled, so to speak. This would be a chaotic and unfocused use of attunement or similar idea. Analogy: Mages would require sight to "write" patterns with their mana, barbarians don't need to see to rip up the "paper", kick over the desk, and stomp it to splinters.

What This Means:
This allows for several expansions and modifications. Base Barbarian magic resistance would be higher than with MUs or other NMUs, but could be increased either in a binary way (>RESIST HIGH, >RESIST LOW) or as a continum (>RESIST 80), but beyond the basic level would sap Inner Fire/Attunement/Spirit/Fatigue/Concentration (I could see arguements for any single or combination of those). Likewise, the Barbarian could give up causing general mayhem and concentrate their efforts on a single target, sacrificing protection from other magics, but making it much harder for the specific target to use magic at all. In theory, it would also be effective against magic items used by the targets.

The Short Version:
Mages use their training to form mana streams into spells. Barbarians use their training to "shake" those streams to unravel the spell matrixes.