I follow about 100 projects in this space "github for data" and haven't yet seen a breakout hit. Yours looks like it has potential. I like the simplicity and the "objects by file extension". Lots of these sites I think get too complex too quick.
At the UH Cancer Center we routinely deal with datasets in the TB - PB range, and that type of size definitely makes this problem qualitatively different. Your splitting of the storage (S3) from the front end is the correct technical decision, IMO.
I've worked in this space for about 10 years. My open source project is called Ohayo, and I used to try and do both front end and backend, and then similarly decided to drop the data storage backend and instead focus on my strengths, which is front end exploratory data analysis.
I think adding a "quilt" keyword to Ohayo, and access to the Quilt datasets directly in Ohayo may be mutually beneficial. Ohayo is just a single dumb web app (no online storage, no tracking, full program source code are stored in the url) and pulls in data via HTTP. Here's an example program that shows the post history from the 2 quilt founders on hackernews: https://ohayo.computer?filename=hncomparison.flow&yi=~&xi=_&...
We use Vega for visualization. You could imagine allowing fast simple EDA on these Quilt data sets through simple Ohayo links. Ohayo version 14 is a substantial improvement and I hope to ship next week or two, and then would love to add Quilt to the picture.