I’m working on a little idea that blurs the line between SP & MP and I have been thinking about what defines a single player game. I should also mention that I’m only viewing this from the perspective of First Person Shooters. I’m not suggesting that every SP game/mod/level has to have these points but most will.
Here’s what I have so far:

1. You do not play against humans.
2. The action/events are story driven.
3. A set of objectives, generally linear.
4. Possible use of friendly bots.
5. A variety of enemies that you cannot play as (Without a mod).
6. An ever-increasing arsenal of weapons and usable items.
7. Each level has a start and a finish, although there may be more than one route to the finish.
8. The inclusion of side quests that may affect the final out come.
9. Bosses and boss levels.
10. A variety of different playing styles may be required to complete the game. For example it is only possible to complete a level by sneaking passed the guards.
11. Most weapons are exclusive to enemies, meaning you cannot collect them.
12. Ammunition is never needed by your enemies.
13. Items, weapons and ammo do not regenerate after collection.
14. The ability to save and continue at a later date.
15. The story is developed through various means during the game. For example cut scenes.
16. The use of NPC’s. These are never used in MP FPS maps (To my knowledge).
17. A variety of communication methods used to assist in the achieving of objectives. For example readable messages and computer logs.

Have I missed anything?