Sometimes it's the limitations that make something interesting. There are levels in the campaign that give you severely limited dungeon-sprawling capabilities, but it just gets all the more delightful to find something that works out because it ends up taking on an entirely new feel.
Like one of the final missions in the campaign... There's this great honkin' big underground castle built up, and you're wedged into a corner with bedrock and lava all around you. You need to worm your way around, fill in gaps with necessary rooms, and then start stealing rooms from the computer through careful management of imps and the use of secret doors. You learn to take what you can get and make the most out of it, so your side of the map starts looking like this haphazard pit of evil, mixed in with little hidey-holes where devils can creep around behind the walls.
Now, sure, Masterpiece does allow you to design expansive cathedrals with pillars and arches and doors and whatnot, and that's all well and fine (heck, it's what I spent most of my time doing, even in the online games. I very rarely won, but everyone agreed that I had the most awesome-looking dungeon)... But there's a completely different and exciting feel to "making do" with less-than-favorable conditions... Like the map where your dungeon is split down the middle by this great big underground river, and you end up with forges overlooking the murky currents as imps scurry back and forth across wooden bridges.
Beyond that, there's the mapmaker. And once you've unlocked the power within that, you can craft your own damned universes. I made a couple maps a long time back that unfortunately were lost before I ever got the idea to release them... One was an imp-race, where you had to dash your way through a racetrack with all sorts of traps and obstacles as a possessed imp, and another was an old-fashioned dungeon romp in DK style. Take the role of a warrior, priest, wizard or archer and travel through four differently-themed areas, each with their own special boss battle, before finally making your way out of the dungeon and into the glorious light of freedom.
Now, granted, a lot of stuff changes when you bring editors into the mix. But I still feel that there's plenty enough to do in vanilla DKII.