16GB Samsung Galaxy S4 only has about 9GB free for users
Of course, the S4 does offer an SD card slot, so it isn't so terrible, but no one wants to buy a brand new smartphone that has 45% of the internal storage used already. It seems that no one at Samsung has really bothered asking: is this too much? There are some legitimately cool software features on the Galaxy S4, but there are also a ton of features that sound cool on a marketing sheet, but won't be used by most users.
All of those marketing features are adding up and really cutting into the usable space for customers. Fresh from the box, an S4 will have only 8.82GB available to the user. So, if you're going for the 16GB model, definitely look into an SD card.