D2C refers to "Diablo 2 Classic." This means the initial game, from v1.00 to v1.06. This does not include expansion pack (which is referred to as D2X).
Many players prefer D2C, usually v1.06, since they feel it was more balanced and presented a better challenge. High quality items were more varied, with exceptional rares often the best weapons, and there were only 4 acts.
Numerous features were added in D2X, including jewels, charms, the weapon switch hotkey, elite items, more item sets, more unique items, and there were many changes made to skills as well. Most players liked the changes, but for some, D2C remains the best way to play Diablo 2.