LMDE is... Well they don't put as much work into it. They said up-front they made it to answer the question "what if Ubuntu vanished and all we had to work with was Debian?" and it should not be relied on as a solid replacement, so I can't fault them for it. But I can't recommend it as your daily driver unless you're willing to tolerate some things, like RAM leaks that make it necessary to reboot every so often. Also the system completely freezing up here and there was a bit annoying.
Now if Ubuntu really is going downhill, maybe they're going to put more resources into LMDE. That would actually be great, as it is also a rolling-release and I would love to not reinstall my whole system every couple of years. Time will tell, but I can hope.
As for XFCE, I used to love it for older laptops that I still wanted to use. These days though, it is a lot more bloated and I'm not sure why.
I used to love CrunchBang Linux which used Fluxbox, but CrunchBang shut down. Fluxbox was BLAZING fast on older hardware. They say BunsenLabs is a community based successor, but I have not yet tried it.