Let me put it this way. If a computer game (particularly RPG) possesses any of these traits, it's not old-school:
- Has real-time combat, whether including the ability to pause or not
- Relies on the game player's skill to succeed in, as opposed to the characters' skills in the party the player creates
- Features nothing but irrelevant and/or surface-level game background, such as its world's history or the meaning of the story
- Literally sprays loot everywhere, as in a fountain
- Requires the player to repeat the same combat encounter over and over until a perfect sequence or series of moves is completed without error
- Can be completed in less than a month (unless the player has no life and/or has autism)
- Relies on anything other than patience, intelligence, cunning, and contemplative planning to excel in
I think that about covers it.