Ok, not subsets in the original sense but more a way to group a bunch of sets into a so-called “Collection”. The coolest part of this IMO is, that it’s also possible to create collections within a collection which finally makes at least some kind of nesting for sets possible :-)
For each collection you can also create a mosaic of random (or selected) photos of the included sets. This also comes hand in hand with some design changes on Flickr.com and the option to change the way your personal collection looks like (collections on the right, sets on the right etc.). For more details on this one check out the official blog.
Another nice addition to Flickr’s feature set I’ve just noticed is, that you can now finally get e-mail notifications whenever someone comments on one of your photos. You can find the details about this in your account settings -> email -> Receiving activity notification emails. But I guess, this is already an old feature ;-)