at first glance it seems to be missing a write barrier to catch new references from old gen to new gen, which is a very important part of any generational collector
It's a simplified toy, all new objects go into gen0 and then get assigned a reference to any node. So a gen1 node can have references to gen0, but the paths aren't traversed in this example. In the linked msdn they talk about dirty mem blocks to solve the cross boundary references.