Well actually if you read the text of the reviews there's little to complain about, except the titles and summaries suggest that there is. The biggest complaint being that it's construction isn't premium like the metal of the HTC One and iPhone 5.
I wonder how much of the complaining about the GS4's build is to do with device snobbery. The same was true of the GS3 and that still sold pretty well against the better made HTC One X and iPhone.
The truth is that Samsung have had remarkable success selling devices that most reviewers and commentators are quick to write off for various reasons. The Galaxy Note was derided for its size and sold by the bucketload. The Note 2 for being bigger and turned out to be an even bigger success. Even the GS3 was slammed in reviews against the HTC One X, which it outsold in spades.
Samsung aren't aiming at the device connoisseurs of the world. They build phones that reach every corner of the web and appeal to the broadest range of users. There may never be a snobbish reason for using one but that won't stop the GS4 from selling well. Of course 90% of the buying population will be exposed to the marketing push for the GS4. 90% will never have heard of the HTC One. That alone will determine what many people buy.
The GS4 may have a 'lot going on' but that ensures there'll be a feature to entice everyone.What's your killer feature? The camera - well Samsung have put together one of the most powerful cameraphone interfaces, with excellent controls for manual shot creation. Gestures? How about the ability to 'wave' your way through the phone, Minority Report-style?
No-one will use all the features that Samsung have added, but there's a good chance that you'll want at least one or two. And of course those that you don't use can be disabled so they don't intrude on the device experience.
Don't be fooled, Samsung didn't get to the top of the tree by mis-judging its market. My guess is that it has that mix just right once more.