While this is a very long overdue addition, there is still one CRITICAL thing missing from Steam. Proper download management.
You can not limit download or upload speeds. In fact, you have no control over upload at all (apart from disabling cloud saves), so there is no way to limit Steam while it downloads so you can do other things online (apart from pause and forget to unpause later). Downloads being paused while a game is active is infuriating. I know you can work around it with an alt-tab and resume, but even that is unreliable. (and I thought this was 'fixed' a while ago). Finally, there is the small issue of downloading multiple things - there is no queue system either, it's all or nothing.
No small number of people also have these things called usage caps (myself included), so auto-updating games are a pain to deal with, especially when there are a lot of games installed and the downloads are hundreds of MB each. Those few extra MB on a cap is the difference between a normal connection, or being charged $1 per GB or speeds cut to 128kbps.
The thing is, technically, choosing an install location is such a trivial thing, why did it take Valve 8 years to figure it out? Download management is not so trivial however, but come on... 8 years and nothing?
Yes, I just turned a positive (new feature) into a negative. I'm good at that.