The distinction between "system" and "app" containers is almost entirely artificial and down to philosophy. LXC containers can very easily be single app, and Docker containers can be full systems.
Lxd looks interesting, though, in particular in not actively trying to make running your on registry/image server awkward, which appears to be a design goal with Docker, though they are trying hard to describe it as more different to Docker than it appears to actually be. Also if they get live migrations working well (having dealt with live migrations with e.g. OpenVz, live migrations is one of those things that is full of gotcha's, and I've come to prefer to spawn new containers and do quick and dirty ip switches couples with gratuitous arp instead whenever I can get away with it).
Lxd looks interesting, though, in particular in not actively trying to make running your on registry/image server awkward, which appears to be a design goal with Docker, though they are trying hard to describe it as more different to Docker than it appears to actually be. Also if they get live migrations working well (having dealt with live migrations with e.g. OpenVz, live migrations is one of those things that is full of gotcha's, and I've come to prefer to spawn new containers and do quick and dirty ip switches couples with gratuitous arp instead whenever I can get away with it).