Here are some details:

1. Old inventory system is back.

2. Old territories are available again such as Swampland, Cruma Tower, Tower of Insolence, Dragon Valley and Antharas Lair/Nest. 

3. Crafting Recipes will use the old materials such as animal bone, curse bone powder, stone of purity, etc. 

4. The clan system is a 5 level hybrid system and will feature two clan skills. 

5. There won’t be any Academy system on the Classic server. 

6. Old party system is back – 9 man party with bonus XP. The level difference accepted in order to gain XP while you are in a party it’s set to 9. 

7. Seems like PK characters won’t drop items, but will get them destroyed (not 100% sure). 

8. You will need to farm spellbooks again in order to learn your skills. 

9. Old Skill system is back.

10. Old Class Change Sytem with the three part 2nd Class quests. The 3rd Class Change will be available in the next update.