how does [] differ from other constants in F#

In your example, x is an immutable value that is assigned during runtime (but NOT evaluated lazily), whereas y is assigned during compiletime. As an example,

let myDLL = "foo.dll"

[<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>]
extern void HelloWorld()

will not work, because DllImport is an attribute and needs to know the value of myDLL during compilation. However, this will work:

[<Literal>]
let myDLL = "foo.dll"

[<DllImport(myDLL, CallingConvention = CallingConvention.Cdecl)>]
extern void HelloWorld()