What features did you use most on the GS4? Sounds like it wasn't anything that you can't do on the iphone.
the screen made browsing the web a much better experience...
the led notification light blinks a different color for different events using the Lightflow app. I have each inbox set to a different color, so a quick glance tells me what i missed...
swiftkey keyboard...stock samsung keyboard kind of sucks...swiftkey make the accuracy of thumb typing equal to the iphone...turn on "flow" (gesture typing) and its no comparison really....the text prediction works really well, sometimes i can finish a whole sentence with the predictions without typing anything...
xbmc on a 5.5" screen, i don't use it that much as the ipad is bigger, plus each tv already has its own rig, but its nice to have... when constipated...
i like how you can browse for apps at the google play store, click "install", and they download and show up on the phone...
built in data counter, i like that i can see how much data i've used up and which app used how much...Onavo Count suppose to do that on the iphone, but you have to install their profile which renders voicemail useless...at least that was the case for me, maybe its fixed now...
while at work, at where i sit, the S4 and Note 2's wifi pulls a stronger signal from the weak but free wifi than my ip5 did. on the iphone 5 it would connect to the ssid, but i can never load any pages, it always reverts back to LTE...with the 3 weeks that I had the note, I left wifi on since its usable. Now i'm 5 days away from the next billing cycle with 1.2GB still left...
NFC tags...i used to think nfc is pointless because apple said it was pointless...now I have nfc tags at home, at work, in my car, and on my keychain. They were about $1 each. When I get in the car, I tap my the back of my phone to the tag and the phone turns the brightness and volume all the way up, turns off wifi, turns on bluetooth, and launches Waze so I can see the surrounding traffic condition, all in one step. When I get to work, I tap the phone to the tag at my desk and it turns off bluetooth, turns on wifi, lowers brightness, and sets the phones to vibrate. I have the tag at home set up similarly. its convenient...
the cerberus app (which gets a free lifetime license today thru appgratis) is similar to find-my-iphone, except it can do more stuff, like take a screen shot, take a picture with the front or back camera, take a video, record audio, get call/sms logs, in addition to the usual wipe phone/sd card memory, lock, alarm, etc...
widgets, to some they're useless as you can accomplish the same thing thru the app, but i like the Simple Calendar widget where it lists my next few days out on my home screen...
there are more that i can add, but i've already got a wall-of-text going...these are just my experiences...others might or might not feel the same...