From Diablo Wiki
The Diablo II v1.10 patch was a long awaited patch that made massive changes to the game. In addition to fixing countless bugs (and creating a few new ones) it added a tremendous amount of new game content while doing even more to streamline the game code. Modding became much easier with this patch, and the changes made in v1.10 enabled the added features such as the Pandemonium Event in later patches.
The v1.10 patch was authored by Blizzard North programmer Peter Hu, who worked on it almost single-handedly for well over a year. The vast majority of his time was spent cleaning up old D2 code and fixing bugs and other redundancies and flaws in the game code. The changes visible to players, such as new items and monster spawns, are a very tiny % the actual patch content and function.
See the v1.10 FAQ page for an extremely in depth discussion of the patch changes, fixes, and other issues.