I love that they seem to be standardizing subclass progression, but why start them at 3rd level instead of 1st? Even for the 2014 classes that get their subclasses after 1st level I have never actually seen a player wait until then to choose their subclass anyway. They always pick at character creation. Plus, having all subclasses start at 1st level would allow subclass to transform the base class more. Sorcerers could get access to different spell lists depending on subclass. Bards could get different options for their set of always-prepared spells depending on subclass. Rangers could have some subclasses that cast spells and some that don’t. Waiting until 3rd level makes it so that if your subclass is a significant part of the character concept, you have to spend two levels not playing that concept, at least not to its fullest extent.
I'm a big fan of the standardization too (though as
@TwoSix mentions, it might not be consistent across class groups or the classes we haven't seen yet). I also like the idea of starting all subclasses at 1st and allowing them to make large changes to class features. I'm generally in the camp that adventurers should be competent and able to do their job from the start of play.
I can think of a couple minor issues in addition to the larger ones others have mentioned about multiclassing and frontloading character creation decisions:
Too much complexity at 1st level--in a lot of cases, you can't give a PC its subclass features without giving it its class features first, i.e. totem barbarians need to be able to rage before they can get their totem spirit feature. This will naturally make 1st level PCs stronger, but also more complex at 1st level, especially for subclasses that a lot of features to start with. Not a huge deal, and maybe avoidable, but worth considering.
Cludgy information design--the current class descriptions, which shunt all subclass descriptions to the end of a class's section, would be quite awkward if subclasses started at 1st and changed large parts of the core class, such as ranger spellcasting. I can't off the cuff think of a way it could be organized which wouldn't--on first reading--force you to comb through a boatload of rules before deciding what to play. Again, not a huge issue, but it'd need to be addressed somehow.
Because of multiclassing cherry picking subclass abilities.
In principle, this is an issue with the multiclassing rules and not the subclass rules. Considering that they just moved the multiclassing rules directly into the class descriptions of the expert classes packet, this is something the designers could address directly. I'm not sure that any solution to the issue is a home run, but there are lots of ways it could be worked around.
I like not having every decision about my character be set in stone at level 1. And I love new players not having to make one more major and consequential decision about their character at level 1.
If subclasses were picked at 1st level, it would probably be a good idea to have an option to change subclasses at some point, like the Tasha's options for changing fighting styles and pact boons at ASI levels--maybe just an option to change subclass at level 3 if you have buyers remorse.
I don't even think clerics should get their subclass at lvl 1. Make them prove themselves to their deity a bit first.
Well that comes with some fun metaphysical implications, eh.
Just imagine... undifferentiated wielders of divine power getting to pick which god's VIP club to join after they've killed and taken the stuff of enough goblins (at least that's what the mechanics would say, prove-yourself lore notwithstanding).