I liked MMX, however I think the most fundamental flaw with the game compared with the other, better games in the series is that it's too slow.
Like wat. The fundamental flaw of MMX is that some of the absolutely basic things deciding the enjoyment of a game of this kind, mostly character/party building and itemization/looting are bad.
The best thing you can say about it is that it's a game that by default makes you try very hard to like it - party- and turn-based blobber that is a PC exclusive and a continuation of a revered PC rpg series to boot. It also offers some challenging fights with disregard for modern design ideas like difficulty scaling, though it rarely does that through clever combat mechanics or encounter design but rather by bloated numbers and war of attrition, with enemies applying huge amounts of damage and you trying to outheal them.
The character system is just a festival of bad design. Like introducing dedicated ranged classes with ranged options shit to the point of being 100% ignorable, then giving them melee options that are incomparably more potent to compensate for that and then topping it off with stuff like class skill that makes ranged attacks melee. Misguided much? Dedicated defender class that cannot defend shit outside of encounters with single trash mobs and so on. There are literally two classes in this game - melee dmg dealer and a caster (with some extremely original variations like healer or debuffer). There's even some completely crpg 101 stuff missing, like some sort of a backstabbing/poisoning rogue class. The party building amounts to ingenious ideas of "I'll have some fighters with strong attacks and a mage to heal and buff them, derp". It's so boring and inept that even Josh Sawyer could (possibly) come up with something better. Then there's smaller stuff like introducing tons of secret areas and only one character able to discover them (with a spell that need to be recast to boot). I could go on.
Itemization - there's nothing to discuss here, really. It's on a level of an average budget h'n's, which is criminal for a game that MMX should be. I think I've felt more excited about finding a unique item in Skyrim than I did in MMX, which says everything.
The above things are the biggest flaws, but it's not like the less important stuff is good either. Lack of charm and soul that were always so great in all the M&M games - the graphics from the horrible nu-HoMM, portraits from a cheap american comic book, no paper dolls. Technical mess of a poor looking game that runs average at best on pretty much any PC, often stutters during movement and causes temperature spikes. Loading times out of hell. Yada, yada.
Yeah, I wanted to like this game. I wanted to like it very much.
I still struggle to understand why people hate M&M X so much.
I don't think it's a hated game? It's just one that promises quite a lot and then fails to deliver. Its flaws are also very in-your-face and already discussed many times, so it's not like it's something to ponder about.