You are never to young for...what was this about?
Common ground:
Charecters defined by:
-Six abilities
-Level
-Class
-Race
-spells/powers
-equipment
-Alingment
-many specifics of the above
Monsters
-many specific monsters (at least by name and general charecteristics)
Treasure
-coins, gems...
-magic items
---'+' system, name, some general charecteristics
Funny shaped dice
-d20 to hit, others for damage
Armor as damage avoidance
HP
XP (for defeating monsters)
Hybrid medieval/rennasaince, high, sword & sorcery, gygaxian fantasy
Dungeons
Traps and tricks (general concept, some specifics)
Most interesting things you encounter, you fight
XP, treasure, do gooding, butt-kicking, usually all together, as motives
I am sure there are more.