The first disappointing reason is represented by the announced cool hardware and software features, of course, based on rumors: 3D, unbreakable screen, eye scrolling, wireless battery and many other Sci-Fi specs.
Galaxy S4 Specs
Anyway, it's a good smartphone, but too far away by the rumored announced specs. Too bad! Its specs seems very closely to S3, and maybe in the next months the new Samsung Galaxy Note 3 will enjoy us with some hardware specs, that for now it looks to be a little bit from a near Sci-Fi era.