The metal case just didn't sell me, though I like the color and feel, but mine goes in a case instantly anyway. Build quality? Can't say I can see much difference there either, nor could the reviews. 32GB on board would have been nice (S5 can be had in 32 also, but at additional cost), but I already have a 32GB SD in there and it supports 128GB IIRC. Unless you can push the app or data to the card though, that doesn't help, and my one app that uses my space won't let me push my music to the SD. I was not dissatisfied with the camera in the S4, but the better battery life and better camera were what sold me on the S5. I am not as excited about the 5 as I was about the 4, but still pleased with it. I think I might have preferred the less "futzed with" Android on the M8 though
Even though Samsung has done a little less "samsunging" to the 5 than the 4, the apps are still clearly geared toward their own, and those are the ones I replace immediately in many cases anyway.