Also, If I suffer a hard drive crash and lose my wallet files, I shouldn't also lose access to all my stored files....
Right, currently you need a backup of both your wallet and the /renter directory (where all the .sia files are).
If I want to use Sia as a long term archival database (and I actually do), I sort of want to stash my files and forget about it...
In v0.5.0 coming soon you will be able to set your own period. So, you could increase the default 6,000 blocks to something much higher.
Running the client regularly, however, ensures the redundancy of your files on the network. As hosts come and go, redundancy naturally drops and the client is set to automatically repair files. If you don't run the client you loose redundancy and in the worst case your files become irretrievable.
Using Sia as a folder in your OS, in a way that hooks into the file system in a proper low level
There is a Gnome shell extension that sort of does this. Eventually, I would expect other OSs to follow suit. What you are asking for is all possible with the Sia API.
Not having to upload the file once for every redundancy factor unit.
As the number of participating hosts grow the need for redundancy falls. Basically, down to x1.5 with about 50 hosts I think.
In-client Shapeshift.io integration.
ShapeShift does not currently support Siacoin. I have asked them N times to consider it, but I guess they receive hundreds of requests like that per day.
In the same vein, a commercial service running on Sia that lets people pay in btc or fiat.
Absolutely! I'm sure there are 3rd parties working on this as we speak (type).
All great suggestions me think!!