So currently (now that there are Symbols in JS), the order is: array-indexed properties first (integers), then string-key properties, in insertion order, then Symbol-keyed properties in insertion order.
The reason for the funny behavior in treating integer keys differently is that property keys are always treated as strings, so obj["3"] and obj can't be distinguished, and arrays are also ordinary objects, so setting obj was made to do the same thing on any object rather than special-casing arrays and non-array plain objects...
Note that special-casing arrays is precisely what implementations used to do (i.e., for non-Array objects, the order in most VMs was simply "properties in insertion order").
V8 was the first JS VM to stop special-casing Array (and this was done before Chrome went public, and was the behaviour in the first Chrome beta). It turned out that virtually no websites relied on "array index" properties appearing in enumeration order (there was some breakage, but it was relatively few and far between, and believed to be worthwhile for the gains in cache-hits when accessing properties), and this also allowed the more compact array representations to be used for them.
In retrospect if something was going to be standardized I'd have preferred it to be the older behavior which was simpler to explain, but so it goes.