It fixed a number of problems, of which the most serious was the Gharik's Lab Key bug. The problem with that was that once you opened the door, the key vanished. This shouldn't have mattered, because the door was open, but unfortunately, if you left the area – to sell the goodies you'd found, say – and then went back to complete the dungeon, you would find that the door was locked again.That was cured by the v1.1 patch. I can't remember what else it cured, but most of what it didn't were fixed by the v1.2 patch. Unfortunately, that introduced two new bugs, which made it impossible to open (a) the vestibule door in the Shadow Guild in the Frozen Highlands, and (b) a door in Castle Kriegspire which prevented you from completing one of the local quests.
Mok's MM6 patch cured these bugs (and probably others), and also hugely increased the compatibility of the game engine with the more modern versions of Windows.
However, the Gharik's Lab Key bug is easy to avoid once you know about it, I still have two computers which can run using the Win98 operating system, and I still play the original v1.0 game.
