Also I really like that they are bringing Fighting Styles back so that
every class has a subclass choice mechanic.
This must be viewed together with feats (bundled into specialties or similar) replacing prestige classes and paragon paths.
It doesn't really matter what the mechanic really is or how they call it... the whole point is essentially that of diversifying characters with a
progression following non-generic character concepts, opposed to diversifying with
small building blocks.
In 3e we had prestige classes for progressive concepts and feats for small building blocks, while in 4e we had paragon paths instead of prestige classes, it's really not bad at all to try something new. And the idea they are trying now (let's hope it works) it sounds like crossing the boundary between progressions and small building blocks, by arranging building blocks into progressions (or alternatively, breaking down progressions into building blocks). This is just
great because the amount of flexibility will double, since you can customize your PC by the building blocks, or you can choose a progression, or
any mix in between. It still leaves the designer the same very large room for writing splatbooks like in previous editions, and the same room left for the DM to write her own stuff, but it gives players an unprecedented level of freedom (provided their DM allows specific material, of course). It avoids situations where for story reason or for strategic reason you pick a prestige class but you have to clutter your character sheet with abilities you're not interested just because the prestige class is totally unflexible. It allows for two PCs to enter the same specialty (or whatever will be called) let's say Arcane Archer, but make that choice work for both a Fighter and a Wizard because they will be able to swap feats that don't work well for them instead of buying a fixed progression.
But at the same time, subclasses will be a simple way to provide variety
within each class. As mentioned by Mike, it's also quite possible to pick
one subclass, the simplest or most iconic, and put only that one into Basic by default. Subclasses make every class intrinsically flexible, working like 2e kits. They also have a large potential for designers to create material for supplements and campaign settings, and for DMs for their homebrew world.
I'm wondering what proficiency means. With weapon proficiency I can still use a weapon unskilled, I just take Disad. Will that be the case with other proficiencies as well? - I guess so.
This could be a good base rule (IIRC the disadvantage from lack of proficiency cannot be even negated by another advantage). Characters could always be allowed to try, but the disadvantage would make it useful only in emergency situations, because if used on a regular basis it will really yield bad results most of the times.
Overall the new Background concept is even better than before...
I liked the simplicity of Background = 4 skills + 1 trait, but it has its limitations: for once, standardizing to 4 skills means sometimes you have to leave out a fifth or even sixth skill which should would have really made a lot of sense in that background, and other times you have to toss in a skill that is not really needed except for reaching 4. The current packet is still full of backgrounds with unreasonable skills!
Making Backgrounds a "packet lunch" with non-standardized benefits makes them really free from design restrictrions!