The biggest change is the additional support for integers.
That was fast. 5.1 lasted a lot longer, with only middling uptake for 5.2 from my perspective.
One sad note about the integers: http://marc.info/?l=lua-l&m=137320751517046&w=2
I’m not entirely sure how integer support helps. I find the existing automatic support for integer valued floats is almost always satisfactory. One time I had a bug because I was expecting C style rounding for division, but that was 1) my fault 2) still not changed because they kept floating division. If I knew to use //, I would have known I could also use floor.