Can we PLEASE not refer people to penlight anymore? It's half broken, poorly documented, and poorly maintained. It drives me crazy to see it referred to all over the place as a good example of anything.
That being said, this is FANTASTIC. Due to the nature of lua it may well always lack a good community but thankfully people seem to be trying.
Author/maintainer here - Thanks for the kind words! Lua's nature is definitely a bit prohibitive to a tight community, so part of my intention with this list is to provide an overview of what's out there.
As far as penlight goes - feel free to open an issue or PR to discuss any inclusions/exclusions. I haven't actually used it myself, but as a fan of microlight I decided to include the big brother. Everything can be reconsidered.
The user base is fragmented (version 4, 5.1 (LuaJIT) vs 5.2), as Lua is usually used as embedded language (alongside C, C++, ...) for video games (World of Warcraft, Far Cry, ...), applications (Adobe Lightroom, ...), etc.
And the http://lua.org website is a bit too minimalistic to offer community features. The project only hosts a mailing list and a old-school wiki. The additional libraries are/were hosted on http://luaforge.net , an outdated sourceforge clone. The documentation is static and the development process itself is not so open. The Lua language standard library is lacking functions almost every other language ship with their default package (so everyone has to either find & download additional libraries or reinvent the wheel).
Other languages like PHP have a documentation that allows the community to add comments and code samples.
Not necessarily proprietary apps, but a wide variety of use cases; Lua is embedded in all kinds of different stuff and used for game dev, webdev, embedded scripting, WoW, configuration, and much more, whereas JS, for example, is used in just two main (related) cases. This makes Lua's userbase rather fragmented across these niches.
Penlight is the closest thing there is to a Lua standard library (loosely modeled after what exists in Python). It is maintained by Steve Donovan who is one of the most active members of the community and the author of http://www.luafaq.org/, LDoc and lake.
Having contributed to Penlight myself I can say it is certainly not poorly maintained compared to most other Lua projects.
I agree, Penlight is not the best at everything it does, and it probably does too much (which is why there is microlight), but it is still a tool most Lua users need to know about.
PL is an enormous library so its really superseded by a few different libraries. The stuff I used most often were the functional tools and Moses (https://github.com/Yonaba/Moses/) blows the doors off of PL in that regard.
That being said, this is FANTASTIC. Due to the nature of lua it may well always lack a good community but thankfully people seem to be trying.