The keycaps are thicker, manufactured in Germany, and are higher quality in terms of fit and finished when compared to a cheaper set thats produced by Tai-Hao. GMK sets are typically designed by members of the community, rather than mass-produced, so that also contributes to the price tag. Since it's ABS it will shine over time, but its worth it if you appreciate the quality of design these sets have.
You don't really get backlighting with these sets, and generally people who are really into the hobby don't really care much about having backlit caps as much as they care about the design. I think backlit caps interest a lot of people at the start (myself included) but then they gravitate more towards keycaps designed with different colorways or themes in mind, and having backlighting typically would work against the overall design. I wanted to touch on this part, mainly because I didn't initially understand why someone would want keycaps without backlighting - I mean how else would I be able to appreciate my Ducky's beautiful RGB backlighting? But I took the plunge, and never looked back.
...why did I type such a long response? I don't know....