I had the Galaxy S2 and Galaxy S3, thoroughly enjoyed both, and when my GS3 broke I chose to wait and get the Galaxy S4 over the Xperia Z which was already available and Htc One. Reason?
-FullHD Super AMOLED (love the deep, vivid, eye popping colours of AMOLED)
-SD Card slot (I'm a media junkie, I have tons of music, music vids and movies)
-32GB Internal (Samsung South Africa skipped the 16GB S4 and brought the 32GB)
-Battery (2600mAh,Samsung have been great at this, if not the best in Android since the GS2)
-Octa Core (a whole new powerful CPU compared to my GS3)
-TouchWiz (the sheer amount of functionality in TW from Multiview Multitasking, Multi item clipboards to the S Apps like S Memo which I use a lot!)
-Build (the GS4 is just so compact and slim compared to other 5inch smartphones and offers great ergonomics)
-Updates (Samsung seem to be the best ,S4 even arrived with the latest 4.2.2 out the box at the time)
-Camera (Excellent 13MP Camera, its no PureView beater but its good enough)
-Wolfson DAC (incredible sonic experience!)
-IR Blaster
-The gesture and airview features (some gimmicky some very useful, all fun to use and show off)
The entire package was just perfect for me and I simply cant complain, had this beast since launch day and it has been brilliant!