Are there any real measurable cons to the S4? I guess it's subjective in some cases.
TouchWiz could be considered a con. It adds features, but some people dislike it. You can use another launcher, but you can't truly get rid of it without rooting and flashing a different ROM.
The fact it doesn't run pure Android could be considered a con. My unlocked/unbranded S3 is running 4.1.2 which is the latest version available for the S3, while the latest version of Android is 4.2.2. It pissed me off that I bought an unlocked phone specifically so I wouldn't have to deal with the network's bullshit when it came to getting updates, yet Samsung rolled out updates to branded phones first in the UK, which is the first time that's ever happened with any phone AFAIK.
The plastic casing could be considered a con. I don't find the S3 feels cheap though, as some people claim, and it's been pretty durable as I've dropped my phone a few times but there's still not a mark on it.
I dunno about battery life and the camera. The S3 battery life is OK - I get a full day out of it - and the S4 battery is 500 mAh greater in capacity, but then it has a bigger, higher res screen to power as well as higher specs pretty much all round.
I love the S3 and I'd swap it for an S4 in a heartbeat, but I'm not going to buy one. I'd just be doing it so I could have the latest shiny, rather than because I actually need what it offers over the S3.