Here are a few:
- Night was actually dark.
- Enemies had no leashes.
- Zones had enemies of very disparate levels.
- No radar or map.
- No quest log or quest indicators.
- Death was a tragedy, not a minor setback.
Running through a pitch-black zone full of aggressive enemies that will never stop chasing you, with no map or radar to guide you, while knowing that dying means a 30-minute run back to your corpse, is more challenging than anything I have seen recently.
I am not claiming this should be revived, but it was a challenge.
Actually, I would love to see no quest indicators, since that meant you had to actually pay attention to what NPCs were saying and learn about the lore.