any links why newcomers like me should be impressed ? Whats the advantages ?
That's just cleaning. The old class builder was an incredibly complex piece of code that is now replaced with the Slot class builder.
The later is based on slots and is much more understandable and thus much easier to maintain.
It's been integrated some time ago thanks to Martin.
Slots are the work of Toon and Camillo and enables a lot of useful features:
They are based on the reification of class layouts (variable,weak,fixed and co) and instance variables (slots).
Slots are an abstraction added on top of low-level class "fields".
It means that one will be able to create a customized slot class that define the logic to read and write a slot:
Object subclass: #MyClass
slots: { x => MyFancySlot }
category: 'Playground'
Then when a method read or writes x, the logic you defined in MyFancySlot is executed.
All that still need to be absorbed by Opal so it's not available right now but it will (Pharo 4)!