Question: When you download the Diablo 3 client there's a "playable" stage and an "optimal" completion. I'm assuming "optimal" includes stuff like higher-quality textures but I could not find any precise details about it. What are the precise differences between the "playable" and "optimal"?

And if my computer is on the low-end of the system requirements anyway, is it even worth it to wait for the "optimal"?

Answer: 
Much like World of Warcraft's launcher, "Playable" means the launcher has downloaded sufficient game code and initial media for you to launch the game and start playing it.

Unlike "available" where you run into "loading walls" where the game client struggles to keep up with downloading media necessary for you to play, "playable" means it has downloaded enough data for you to play without hitting any "loading walls".

> 
> Setup (Red): Core game content is being prepared. You cannot enter the game at this time.
> 
> 
> Available (Yellow): Major game content is not completely applied. You can play, but your game experience will not be ideal.
> 
> 
> Playable (Green): Final game content is almost complete. You may experience a few issues with playing at this stage.
> 
> 
> 

The "Playable" includes high resolution textures and other low priority flair:

> 
> Q. How is data assigned to each stage?
> 
> 
> A. Data is assigned to each stage based on the relative need for that data. Red is required to get into the game, Yellow is data needed for baseline gameplay, and Green is for variants of base game data (higher-resolution graphics, alternate sounds, other flavor data). [[1]](http://wow.joystiq.com/2010/10/11/new-streaming-launcher-available-soon/)
> 
> 
> 

Therefore, "optimal" means it has completed downloading all available content and will not impact your play experience with background downloads.

(Information is from World of Warcraft's launcher, which uses the same technology)
For a much more in-depth answer, see Resorath's answer.

Simply put, you will be *able* to play it during playable, but you will experience issues with graphical and possibly network quality if you choose to play [it will continue to patch as you play].

When it is in the optimal state, that means that you can play without your gameplay experience being affected too much, if at all, by the update process.

The same applies to all of the games associated with the Battle.net launcher. Starcraft 2, World of Warcraft, Diablo 3 and Hearthstone all follow this update approach.