After many playthroughs, I remain convinced that the single biggest issue with MMX is that only a single class is able to learn Dark Magic at all.

There are a few useful Dark Magic spells, and I'll list them below in increasing usefulness:

Dark Vision: improves visibility in darkness. Nice, but other spells can replicate

Shadow Cloak: next attack from any enemy will miss. Useful, but can live without, and after a certain point you'll no longer bother to throw it up

Darkness Ward: resistance to Dark Magic. Always useful, but once you get to the point where you really need it, can be replicated with other spells (although not as strongly)

Whispering Shadows: the only way to detect secret doors. Some of these are necessary for completing dungeons/quests, but can be replicated by scrolls, although this is only really possible for playthroughs where you already know the location & can trigger the scroll at the right point. Otherwise, you'll waste all your money buying scrolls for this - if you can find them at all. The ability is granted as a latent ability a little more than halfway through the game, which renders the spell redundant from that point, and it can be replicated by a specific NPC before then, although it's almost impossible to obtain this NPC at lower levels. Overall, not fatal, but makes for a far less enjoyable game having to go back to previous dungeons to complete once you finally obtain the skill via other means

Sleep: this is, without question, by far, THE most important crowd-controlling ability in the entire game, to the point where you will be wasting plenty of time reloading without it. Unless you're attempting a very specific playthrough, you need this spell, or you need plenty of scrolls to replicate it, which are hard to find. In short, the vanilla game is virtually unplayable without this ability, which means it's virtually unplayable without a character able to learn Dark Magic, which means it's virtually unplayable with a Freemage (the only class able to learn it)

And THIS is the single biggest factor that significantly reduces the enjoyability and replayability of MMX. One of your four characters pretty much always needs to be a Freemage, unless you want to plan out a very specific playthrough. And it's such a shame, because virtually all the other classes allow you to put together a range of interesting and varied parties. But once you select your Freemage, that virtually automatically selects all your other classes, based entirely on what you still need after selecting the Freemage.

If even ONE other class had the ability to learn Dark Magic - even just to Expert Level (what you need for Sleep) - the entire game would have so many more options. Worse, because of the engine, promotion quests aren't even available unless you actually have one of the necessary classes in your party. So if you keep playing the same party, you're even restricted to the same quests, and the others are unplayable. So until someone mods a class to learn Dark Magic to Expert level, this game is pretty much the same playthrough every time.