last playtest packet of the year comes out today (or it might already be out). On the other hand...prestige classes.
Prestige classes, as I knew them anyway, were alternate classes in 3rd Edition that you had to meet certain prerequisites in order to take. For example, if you wanted to take levels in assassin, you first had to be evil, have 4 ranks in Disguise, 8 ranks in both Hide and Move Silently, and kill someone for no other reason than to join "the assassins".
They tended to evoke concepts both broad and narrow (spellsword and halfling outrider), some existed to make you better at something (weapon master), while others expanded on something you could do (force missile adept) or let you do something completely different. Sometimes the requirements made sense, but sometimes they seemed to conflict or exist only to force you to wait until a certain point to take it.
Paragon paths in 4th Edition were nothing like this. All of the ones that I remember required a certain class of power source to be sure, and some might have needed a certain race or alignment, but that is it. Hit 11th-level? Okay, pick one. Do not like them? Then you can still opt for paragon multiclassing. You do not stop growth in your class, and you do not sacrifice anything for them.
The goal of having them represent "interesting" bits of the game world is nice, but I recall that being the initial goal of prestige classes and, well...that did not work out so well. There were prestige classes tied to organizations to be sure, but most seemed to be there to fill in gaps that a core class should have been able to do. Things like a fighter that could take hits for her teammates, a fighter/wizard combo, or even a pyromancer.
Honestly I kind of like how Dungeon World does things. If you meet certain requirements--none of which involve taxing your level-based decisions--you can start taking moves from another class. This works because Dungeon World does not have scaling like, at all, so you are not going to be a 10th-level fighter taking her 1st-level in wizard, only to realize that popping a monster for 1d4 + 1 force damage does not mean anything. In 4th Edition this would be like burning a feat to multiclass, but letting players take powers however they please (instead of spending more feats, and only getting one power per feat).
Fortunately D&D Next seems to have toned down the scaling quite a bit (though we still need to see multiclass rules to determine how well things mix and match). All they need to do now is avoid taxing character needlessly, and try to ensure that the "prestige" bit is more interesting than making you better at attacking while mounted or letting you change your psychic blade into a bow. Personally, if that is all the prestige class brings to the table, that it could be folded into the core class.
The saving grace is at the end where Mearls states that they will both take a modular approach and build them from a comprehensive list of mechanical options (as well as show you how to break them down and rebuild them). Giving Dungeon Masters very clear, concise tools to do their own thing sounds much better than just sticking to the books, or hoping that Class Splatbook 7 has an appealing option for you. I also like this approach because then maybe you could more easily swap out class features as part of a reward system, or even allow a player to better customize an existing class.