One thing I though about concerning proficiencies is how they affect a multiclassed character. The profiency, once it is gained, scales of character level. Applied to weapons, spells, and skills, this will make Multiclassing characters way more attractive. No longer will you have to hamper all of your abilities just to multiclass.
Additionally, depending on how feats are gated, this too could make things a lot better for multiclassers. I'm hoping we hear more on the subject.
We have no idea how multiclassing will work there's a blog that will tell us more later.