Great Post DaveVoyles.
Although I would like to jump in clarify the size issue - default engine Javascript Shipping “binary” is around ~10mb ( it has increased over few iterations of the Engine, although a custom build can cut down the size even more) when compressed. the code part makes it the SMALLEST of any platform UE4 supports.
all modern browsers support gzip, there is no reason at all to not serve compressed javascript.
That being said, we have have listened to our users, as of our upcoming 4.9 release we will emitting out compressed javascript from HTML5 packaging. Also, we will be shipping the SDK with the Engine, which makes HTML5 export trivial with no configuration needed which is currently required for current releases.