How much of the final product will be like the playtest? I mean, the two can't be very similar because if they were, then there would be no need to even buy the books. You already have a free copy.
The SRD in 3e era didn't prevent it to be a huge commercial success, so I don't think the above is true. Even if the final product is the same (which it won't), I wouldn't worry.
Anyway, I expect the final product to be significantly different at the lower scale: actual numbers, bonuses, x/day, and any details of a mechanic or a spell can change in the next few months. Class features may move up/down in level, be shoved into a subclass or viceversa, and some will probably disappear and be replaced by something new.
But at the higher scale, the game won't change except maybe a couple of things, that might revert to previous iterations (but I cannot guess which ones). For example, I don't expect any change to how combat and spellcasting work, to multiclassing rules, to the mechanic of skills and feats, to rules of proficiencies, etc.
Now when it comes to
content of the books, I would really like to know what they plan to include. I would expect the core 3 books to have at least all the stuff shown in the last playtest packet, but that is not a guarantee. They might decide to scrap some parts off, e.g. the exploration rules, but I hope they won't.
And more specifically in terms of character material, I would certainly like to have more than just 2 subclasses per class (which is the case with many of them), and at least a triple amount of feats because they are very, very few... if you want something else than combat feats or feats that grant spells, you end up always with the same few feats. Unfortunately IMO the current packet is quite large already, and once you factor in the artwork there might be no space left for additional character material in core, especially if they want to include Sorcerer and Warlock.
I'd rather have few magic items than few feats, because magic items in 5e are allowed to be more free-form which means a more relaxed job at balancing your own custom magic items, but IMHO if you make your own feats you still have to balance them carefully with the existing ones.