Threads you follow have all been erased, during the migration, the new functionality here is called, "watch these forums" , you will need to check all post and threads you want to follow again . The notification is much better, see here than an overview in the top menu of the threads and forums you follow - Screen Shot 2016-03-02 at 18.07.51.png\ Screen Shot 2016-03-02 at 18.11.08.png SJ
No all existing content is identical , only because of the two system differences we couldn't import subscriptions. However as @Superjoint highlighted the new system is way more advanced then before.