@ZombieGenesis I think that's true of any Superhero game, and a lot of RPG's in general. Simplicity does not in itself make a system easier to use for some players. For me, systems like FATE or Cypher just feel a bit wonky and loose for superhero games because of the sheer, nearly-unlimited potential for what you can encounter or create in the genre. Or maybe I've just had bad initial experiences that soured me on them; either or. HERO/Champions and M&M feel more 'right' to me by having more defined scales and direct comparisons and measurements (5pts in HERO is 1d6 of effect. Each point in DC Heroes is twice as powerful as the previous point. Etc.)
Superhero games in particular walk a very fine line between simplicity and complexity. If you get right down to it, both HERO and M&M are fairly simple systems rules-wise; roll, add or subtract modifiers, compare result to a target number to determine success or failure. Simple. The complexity comes in building characters and powers. If you look at a game like, say, D&D (any edition), character advancement and powers, spells or otherwise, are already pre-created for you, so there's no real need for a 'tool box' to allow you to create your own powers or abilities. Making minor changes (swapping out Fire effect for Cold on Burning Hands to create a spell that shoots shards of ice instead of flames, for example) may have negligible effects on the system. Creating new paths of advancement or classes can lead to problematic imbalances in power scale (remember how broken the Gladiator class in Dark Sun was? I'm a Fighter that can use any weapon without penalty, even that brand new homebrewed weapon the DM just created! Who cares if I'm a slave; that's just about the default state for everyone here. And who cares about armor? It's 50 Bazillion degrees out and the whole planet is a desert!). Marvel FASERIP is the one system that managed to balance that walk beautifully, but all the powers are pre-built, leading to the (admittedly rare for FASERIP) occasional situation of 'What if I want to be able to do X?' where X isn't covered already by the system.
Superhero systems that get a 'complex' rap are usually systems like HERO and M&M where the bulk of the book is a customization toolkit that tells you how to create powers, skills, abilities, etc. And, several times, they don't do very good jobs at explaining how to create what you're thinking of (HERO actually does a decent job of this, with examples for just about every power in the core book). To be fair, both HERO and M&M also put out splatbooks (UNTIL Superpowers Database/Ultimate Series & Powers Database Series, respectively) that contain numerous examples of how to recreate powers and things you see in the comics with the game system. But, these aren't in the core rules. In my experience, a lot of people look at the size of the rulebook, especially MU players used to freeform, and their eyes glaze over thinking that the entire book has to be known in order to play the game.
Doing systems like HERO or M&M on mush is more difficult because character creation takes a lot of interaction between player and Staff. And Staff also have to see potential issues and feel free to say 'No' (M&M Afflictions, anyone?). It's much easier to be one on one and fine tune concepts in a tabletop environment.
One thing I would love to see for any mush that uses a TT RPG system is a 'Training Room'; an OOC area where players can go and test out the rules and action commands. I think having a set of 'target dummies' that you can practice your +attack or +cast on for an hour or so off grid or when RP is not available would help familiarize players with the game commands and system much quicker. Also, if it's done like the Power Centers in Champions Online (the MMO), players can test new powers/abilities out and examine the effectiveness firsthand, then decide if they want to do fine tuning or scrap the new power/ability entirely before committing advancement to it, it would help cut down on the +requests for 'I want to readjust my power, it's not working out for me'.
In the end, toolbox systems are what I prefer for superhero games, but a lot of times they're implemented poorly in the design of a mush. There's not enough emphasis on actually planning out your character, for both creation and advancement, and that's an important key to HERO or M&M. Mainly, from what I've seen for TT ports to mush, it's either 1) all FC's that are pre-statted, or 2) 'Okay, the concept blurb for your OC looks good, in to CharGen you go! toss the system newbie into the shark-infested deep end of the pool and wanders off to do Staff stuff'