Most cryptocurrency users probably fall into the "using residential or other limited-bandwitch" category, but I cannot think of any other cryptocurrency that needs QoS. I am not well read on this topic so I may be wrong.
Let's exclude coins that have virtually no usage. They essentially exist only on exchanges as speculative pump-and-dump vehicles.
When it comes to the small subset of coins that actually have users, they do run into this. I think we can agree that not having any users/usage is not a good way to address the problem.
The XMR blockchain size is not extraordinary as popular coins go, especially extrapolated forward. The DOGE blockchain, for example, is around 6 GB (I'm not sure if that is the on-disk size or the raw block size, but I suspect the latter, meaning the corresponding XMR size today would be about 800 MB). If you download that or more importantly upload it to other users who are downloading it, you will have major bandwidth issues.
It is a frequent issue for Bitcoin for that matter:
http://www.google.com/#q=bitcoin+qos+site:bitcointalk.org