Well, as long as this post is here, here's my random guesses.
All class progression in features is now through feats. A class feature might get slightly more powerful to remain somewhat useful at higher levels, but you will never randomly get new options from leveling up. All class progression is feats and classes are made of feats.
Since classes are just assembled out of a bunch of feats, multiclassing is taking the feats that make up the base features of another class. The first feat you need to take before any others in a class is the "multiclass X" feat. This allows you a certain amount of access (perhaps even full) to their power list when picking new powers. It also gives you the ability to take future feats that are class abilities of theirs.
Abilities that would be overpowering as one feat are broken into two or more. You might only get 2d4 sneak attack with the first feat but the second raises it to 2d6.
There's also a training line of feats that allows you a small benefit that's in the style of the class, but you do not count as the class for prerequisites. The nice thing about this line is you get an immediate use from the feat. The multiclass feat means you practically have to burn a feat who's sole purpose is to give you access to that class' feat and power lists.
Since any feat can be retrained as far as we know, that means you can also unmulticlass if you wish. This is why you will always be your first class from 1-30. You cannot retrain your base class.
Anyway, yeah. Random guesses 5 and a half hours before we find out.
