Actually, while others have been marveling over the progression, I wonder if we haven't seen some evidence for the "feat-based" multiclassing being the main/only method in the core books from this excerpt.I hesitate to say that with certainty but the text and the progressions seem to indicate that the game either assumes 0 changes of base class throughout or that there's something extremely major we haven't seen involving class changes.
When I mean multiclass, I don't mean "essentially single classed, except for a couple of powers," which is something that sounds like the perfect reason to have a class training feats that give a couple of the powers of a class and that's it. I also don't mean retrains like "Oh, i was a wizard 10, but now I'm a fighter 11 now that I've leveled.." I mean combos like a fighter/warlock which combine multiple classes pretty much evenly and for a good portion of the character's career.
One possibility I just thought of is that multiclassing is supposed to be primarily done on a tier basis. An example of this idea would be a character who wants to be a combo fighter/mage picks up one of their classes from 1-10, then the other from 11-20, then has an epic destiny designed to blend them from 21-30 (I don't know how the epic levels would play out though in terms of class choice). This might be in lieu of a paragon path, for example, or even an epic destiny (which explains the coyness of some of the devs regarding characters without them.) as it is possible that could be a way to add class features and appropriate powers, as the additional features might add onto the 'base' that is already present without taking away the class features of the base class. In that case, it would definitely be reasonable to have the 'multiclass' characters features be slightly less potent than the single-classed character's of the same type, though where applicable, more potent than those of someone who doesn't have the "class" at all. Likewise, while a multiclass path/destiny offer's up more variety in the power selection over the entire tier and enable a character to pick relatively freely from both class's lists, the class training feats might allow a single, one time choice from another class's power list.