I'm not entirely sure "exhaustive" is what I'd call Marvel Heroic.
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all I can figure out about the game is that it's a dice adding game with a story attached.
Being Wolverine doesn't actually give you any benefits at all. Your claws don't do extra damage to enemies compared to people without them. Your regeneration doesn't have any real effect over someone's forcefield.
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The mechanics are so disconnected from the story they create that they don't seem to have any relationship at all.
This reminds me very much of similar comments I've seen posted about 4e, and in my view is mistaken for the same reason: it's not having regard to how the mechanics are actually intended to be used by those who are using them.
[MENTION=87792]Neonchameleon[/MENTION], upthread, used the notion of "fiction first", and MHRP is definitely this: you frame the scene and the action first, and then build a dice pool that reflects that. Your various traits and SFX support the building of that dice pool.
Wolverine's claws help him do damage compared to others, because when he builds a dice pool to attack he gets to add a d10 for his claws; whereas (say) the Punisher only gets to add a d8 for his guns. But the question of whether a given weapon can be brought to bear in the situation will already have been determined via the scene-framing (ie fiction first): for instance, if the enemy is flying, the Punisher can declare an attack, but Wolverine is going to have to find some way to get closer to the flying target.
As @Vyvyan Baterd pointed out, Wolverine's claws also allow doing damage on a reaction (because people hurt themselves punching his adamantium skeleton), and Wolverine's regeneration allows him to use his Stamina to recover large amounts of physical stress during an Action Scene.
A forcefield is quite different - it provides Durability. Complaining that a forcefield and (say) Colossus's armoured body are both mechanically similar is like complaining that, in D&D, a shield, a suit of armour and a ring of protection all grant AC bonuses. D&D doesn't distinguish mechanically between armour protection and shield protection (contrast Runequest or Rolemaster, which do) and MHRP doesn't distinguish mechanically betweeen a forcefield, a normal field (like Captain America) or a tough skin/skeleton.
If you didn't get these things from reading the rules, you might need to re-read them.
The problem isn't that there's nothing interesting planned on a failed Diplomacy check. It's that a skill challenge has a distinct structure. You must succeed in, say, 12 successful checks before you "succeed". Often, it is hard to find an excuse within the game to even make that many checks.
The problem is
exactly that the GM has nothing interesting planned on a failed Diplomacy check, and has therefore framed the challenge wrongly - ie as requiring 12 successes (and therefore as many as 14 rolls) when the GM doesn't have 13 interesting developments to narrate during the unfolding of the challenge.
If the GM can't think of anything interesting to narrate, then s/he shouldn't be framing a challenge at that level of complexity.
In practicality, skill challenges just couldn't be done without it feeling "out of place" beyond a certain complexity level. You couldn't have a skill challenge to "convince the king" of something that required 12 successes because play degraded into continually rolling diplomacy checks. None of the other skills had a very logical reason to be used in this circumstance and any attempt to use other skills always seemed like a leap of logic and that the player was trying too hard.
If the GM wants to run "convince the king" as a complexity 5 challenge, s/he needs to have stuff in mind: like the king asking the PCs to demonstrate their prowess (might involve something other than Diplomacy); or their being some other party invovled (the PCs have to deal with some obstacle other than the king); or complications arising that rely on some other sort of knowledge to be addressed (the PCs have to make knowledge checks); etc.
Here is an example from my 4e campaign of what I have in mind.