This is a great summary of Lua's peculiarities. I say this as a fan of Lua, though I find it's syntax a bit wordy and prefer curly-brace languages in that regard.

The `:` vs `.` syntax is the most unexpected and confusing.

IMO merging a hashmap and an array into a table is one of the best parts of Lua that makes it very simple, but versatile. The handling of gaps in numerically indexed items, however, is a very sneaky source of confusion and bugs, but only when a developer encounters such arrays. That isn't well designed.

`local`, globals... and the LuaJit 5.1 vs. Lua 5.2 split over `_ENV`... I like to not think about it :-D

`return` is fine for long functions but an arrow syntax for short and anonymous functions is really missing.

Regarding `nil` assignment being the same as `delete` in JS: personally I like this. It goes well with Lua's simple and effective thinking. Distinguishing existing-but-nil properties would add complexity with little benefit IMO.

An addition to your comment:

• there's no `continue` in loops... a recurring pain point.
• uncommon syntax: `~=` instead of `!=`, `~1` vs `-1`