@simsmx: What is exactly the difference between dancing and club dancing?
What will happen when a sim become better at hopscotsch?
Again, I have the answers to both of your questions.
1. - The main difference between Dancing and Club Dancing is that Dancing is improved when a Sim is Dancing
solo (only that Sim, and not the "Dance Together" interaction) using one of the stereos (It can be any type. As far as I know, with HELS, there is four of them. But there may be more with Late Night and Generations, I don't know, sorry.), while Club Dancing is improved only if the Sim is dancing in one of the clubs/lounges/watering holes etc., using a dance floor. Dancing has only three levels, but I can't say for sure how many levels has Club Dancing.
2. - When a Sim becomes better at Hopscotch, their win ratio will be greatly increased (like when a Sim becomes better at Chess). I can't say for sure if the animations will improve too, as that may need testing - it's still a new object.
Leto, I am currently working on this spreadsheet-type-of-thingy about Hidden Skills in TS3, I'll report when it's finished.