I don't consider myself an expert, but at a quick glance here are what I see as the main differences:
QNAP on left, Lenovo on right
Intel Celeron J1800 2.41 GHz vs Marvell 1.3 Ghz processor
1GB RAM (upgradeable to 8gb) vs 256MB RAM (Idk about upgrades)
Edited to add: Data transfer on QNAP is ~100mb/s while the Lenovo is ~35mb/s.
2 ethernet ports vs 1 ethernet port
HDMI-out vs no hdmi out
2 USB 3.0 ports and 2 USB 2.0 ports (including front panel) vs 1 USB 2.0 port
Also, I think it's reasonable to assume that the QNAP software is more fully featured and easier to use, but I really can't speak to specifics.
My guess is that for many people, the $50 unit will probably be enough, especially for those just first dipping their toes into network storage, but for those who want better performance, more features, and ease of use, the QNAP could be worth the investment. It was for me.