Items: All EQ items would be wiped out of the database. EQ don't have the same stats as D2, but you could work around that, say an item increases your run speed, just add agi instead. Dex and str are the same. Increase attack speed, just decrease the dly on the weapon. Energy, just add to either wis or int, depending on your class. Alot more, but you probably get the basic idea on the items
How would you go about zoning into nightmare? hell? Say the city is Qeynos where everyone starts in. You zone into the Catacombs from the newbie fighting area, that takes you to nightmare lvl which can't be accessed unless you have defeated Baal. The way it knows if you have killed Baal, is you have a No Drop item that normal baal drops, the zone checks to see if you have that item, and it lets you in. Same for Hell.
Mobs: All the names will change to names that are in D2. Everything scowls at you, because everything in D2 just attacks, they don't wait for you to attack.
Why Qeynos? Mainly because Blackburrow would make a great Den of Evil, Karanas would be awsome for the rest of the quests, and Highpass would make a good Tristram where you save Deckard Cain.
What about the other acts? There are lots of cities, Qeynos would just be Act 1, after you defeat Andariel in Act 1, you will then be given access to Act 2, which could be Freeport. And so on down the line.
Expansions? There are 5 acts in D2, that means you would need quite a few more zones than Classic has. I didn't think of this when making my first post, Kunark might have to be added so you have more zone choices. Kunark would make a great Act 5 if you think about it