LÖVE (a.k.a. Love2D) is a free and open-source framework for making 2D games in Lua.

What I like:

• Straightforward
• Continuously maintained

Where I find it lacking:

• No GameObject/Node/Entity system¹¹¹ Arguably, some games don’t need this; not being based around a GameObject approach is not necessarily a drawback. Still, I find that many non-trivial games end up with their own implementation of GameObject. 
• Can’t publish for the web (HTML/JavaScript)²²² There is love.js, but it is unofficial. 

Tips:

• .love files are just .zip files, so you can easily look at the source code of any LÖVE game.

Alternatives:

• Godot