What are some cross-pack features you know?

More that come to my mind.

• Clothing will dry slower on clothesline when it's raining.
  
• Cats can play with droplets that fall from clothes while they're drying on clothesline.
  
• Another biggest cross pack feature is how your sim can get famous. This even includes other pack careers which can give fame chance cards.
  
• Then there are holidays and tradition. While there's only one tradition that requires a pack (that being fireworks from City Living), activities that complete traditions (Spooky Spirit tradition can be fulfilled by carving pumpkins) and which sims love or ignore which traditions (vampires love Spooky Spirit) is also there. Though I feel like it needs updating as Servos won't ignore grand meals...
  
• Some pack activities build character values, like filling a pet food bowl (raises responsibility) or giving gifts (raises empathy).
  
• Charismatic Crooner reward trait from Salaryperson career helps to build singing skill faster.
  
• We also have some sims who will have certain other pack skills when the required pack is installed. Then we have Bonehilda. She has Pristine reputation and 4 levels in Baking skill. In addition, she's Chopstick Savvy.
  
• Some packs add more items to buy from food stand. Let's take food stands in Tartosa for example, which will have more foods when City Living is installed.
  
• Cloudy weather is the best for vampires as it won't make the sun harm them.
  
• In Dine Out, restaurants can include food from other packs.
  
• Plopsy also allows your sims to sell and buy candles, flower arrangements, potions etc.

I'm trying to leave out cross pack gameplay from Cats and Dogs and My First Pet Stuff out as the stuff pack is DLC for DLC, thus it has a lot of cross pack gameplay for Cats and Dogs. For example, sims can learn the cure for Rabid Rodent Fever from a vet.