I definitely feel the lack of a ternary conditional expression in lua.

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Why doesn't lua let my index strings using s[n]?  I have to use s:sub(n,n).  Perhaps to avoid confusion over whether the result would be a character codepoint or a string?

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Why doesn't lua standard library have a math.round(n) or math.toInt(n) that does math.floor(n+0.5) ?

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Lua's do/then/function ... end syntax isn't friendly to bracket matching in the editor

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Lua (like Python) defaults to a global variable when it doesn't find a variable in scope - including for assignment.  This is a bit troublesome because if you have a typo or renamed a variable you get wrong behavior and no error.

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When you define a function inside another function, if you don't explicitly mark it as "local" it will be assigned to a global variable, i.e.

    function b()
        print("goodbye!")
    end
    function a()
        function b()
            print("hello!")
        end
    end
    b() -- "goodbye!"
    a() -- over-writes b
    b() -- "hello!"

This is not a good thing.