Examining it in a vacuum, the S5 is a beast--top notch CPU, great display, what appears to be a terrific camera, great display, good size battery, expandable storage (up to 128GB 😱). Yes, there are hardware features that I personally feel are half-baked (fingerprint sensor that for all intents and purposes cannot be activated holding the phone one-handed) or gimmicky (heart rate monitor) but are optional and don't take away for the phone if not used. I can certainly understand how this device would be an attractive option for people. In fact, most of my qualms are pretty geek specific so I really didn't expect to see much different than what we actually got today.
My own personal issues with the S5 are threefold:
1) Design--both hardware and software. Physically, I think it's an ugly design. The back panel is atrocious, the camera hump ruins any shot at a clean looking rear panel--everything looks cheap. On the software side, MRU has pretty much hit the nail on the head. There is no consistency to the software design style. It's all over the place and IMO, is also just ugly--cartoonish and dated. And 8GB for the OS--come on, that's ridiculous.
2) Materials--nothing wrong with using plastic, however it doesn't need to be cheap feeling and looking. Both Nokia and Samsung extensively use plastic throughout their devices but you'll never mistake the two.
3) Size--the Galaxy S line just keeps getting bigger and bigger and is borderline phablet. The S5 is much closer in size to the original Galaxy Note (about 5mm shorter) than the original Galaxy S (nearly 20mm longer 😱). I wouldn't have issues with this if Samsung didn't gimp the hell out of the S Mini.
As I see it, Samsung needs to dial in the Galaxy S line, not keep adding to it. Focus on the user experience, overall and optimize the software, tighten up the design of both the hardware and software. The problem is, marketing can't sell this so Samsung is left no choice but to keep adding in whatever they can think off.