I loved pathfinder archetypes, as a quick and effective way to hone in on specific, evocative characters, so this is terrific news IMO.
Being able to pick and chose specific components of the archetypes, however, led to many OP and broken characters, due to the difficulty in balancing all the options vs each other. There were some barbarian and summoner ones that were just clearly waaaaaaay better when pick can pick ala carte, and they seemed to have missed that aspect of balance in their compatibility matrix. The easy way to fix that is to, as they say, go "whole hog". I'm totally down with fighters going down different archetype paths though, but they should really put a limit on ala carte multiclassing like 3 max (or even two! that'd work), I agree 3e was somewhat ridiculous and I don't want to spend 18 hours finding OP / optimal combos between classes where you can exploit their sub-classes fiddly / broken bits (where they might not be broken when taken on their own, but in conjunction were).
The big cheese with 3e/PF was taking class combos that made no sense in the story, or you really had to contort one into existence to justify it to your DM (or to yourself) merely because you wanted some uber powergaming combo. As a powergamer myself, I want sticking to one archetype to be by default the best option. I don't want my Barbarian, like in PF to have little point in levelling up more in Barbarian levels past 11 because fighter gave you more power, or you got your pounce beast raging ability and took your one level of Horizon Walker or the ridiculous crippled Oracle that can't be fatigued. You will inevitably, with splat books, go back to these same broken combos, so best to simply put the default max # of classes to 2-3, and on a dial so that DMs can restrict it less or more, according to taste. What I'd much rather is a game where the core class, if it has drawbacks such as the barbarian rage mechanic in DDN granting advantage to your foes, to not result in everyone taking two levels of Lame Oracle or its equivalent to overcome that. The default class should work, and work well all the way up to the top. You need fun abilities throughout the levels to look forward to, other than your big jump at level 9 and your capstone. Many of the other levels seemed like filler and you were like 5x more damaging once you could "pounce", thus full attacking every round. Sure, they killed the full attack nonsense in DDN, but you see what I mean. I could see some combinations of subclasses and prestige classes between wizard, sorcerer, and cleric giving you uber spontaenous casting with all your spells, possibly happening. And then, of course, everyone will take that.
If it's too easy to cast your spells in full plate, people will take that feat or one level of fighter to gain the proficiency. Is that a bad thing? I'm not sure. A level 1 wizard / 1 fighter is much tougher than a level 2 wizard, and then keep continuing in wizard would be pretty cool too. I can't wait to see how they do this.
They said that two weapon fighting would be independent of class and hopefully independent of subclass (which in Pathfinder, for example, they had similar archetypes for fighter AND ranger, and the fighter one had this ability to get his "twin strike" with two longswords, but it never benefitted from all his other feats...sucked). So anything that you want any fighter type or rogue type such as dual wielding to be able to do, shouldn't be really made uber good via an archetype, to avoid duplication. I want there to be MadMartigan-esque and Conan-esque warriors who, once they get good enough, would wield two battle axes or longswords or bastard swords, but not at level 1. In 3.x and PF, the default feats never made it worthwhile to trade off so many to-hit penalties to use two medium weapons for an extra +1 to damage on average. With deadly strike, there's a good reason to use a higher die weapon now, so hopefully there'll be some graceful way to split deadly strike between your attacks. Evenly, is probably best / easiest. I.e. if you hit with one or the other hand, you get 1/2 your deadly strike, but if you hit with both you get both, which means you'd have a higher average DPR but not a greater maximum, and that only comes into play when you have a lot of deadly strike die to spread around. If you divide by two rounding down, most levels you'd have a higher max damage but lower average wielding a single weapon, which is good. I want the super strong guy, with a good to-hit bonus, to want to take that -2 / -2 to be able to plough through tons of mooks.