1. Fingerprint scanner: will it ever register your fingerprint from the first time?
The more you use the Galaxy S5, the more you realize that the fingerprint scanner would rarely ever recognize your paw's unique signature from the first time. Try it again, then again, then again, and then just when you're ready to give it up, it recognizes it. Do you have the nerve for that many tries, though?
2. Vibration that could wake up the dead
The default vibration for the Galaxy S5 is set to be so strong that it could literally wake up the dead. Well, maybe not literally, but boy, does it get close to it. Luckily, there's actually a solution for that: go into settings -> sound -> vibration intensity, and set it to lower.
3. Phonebook and contacts app still lags, thank TouchWiz for that
Can you believe that TouchWiz still stutters here and there? Even with the newest and most powerful quad-core Snapdragon 801 system chip on board. The slowdown is especially noticeable in contacts app when you try to call someone.
4. UI design is a mix of everything
TouchWiz is evolving, but on this path of evolution it does not scrap old features - it simply accumulates them one over the other, and the end-result is a UI that has more different elements than there are items in a garage sale.
5. Bloop, bloop: nature-immitating sounds can get on your nerves
Bloop, bloop, bloop! Those are the sounds that the Galaxy S5 welcomes you, and your initial reaction might be an 'awww', but you'd be frantically looking for a way to stop 'that annoying sound' in about 10 minutes time. Hey, luckily, you can: go to settings -> volume -> touch sounds, uncheck it, and you're done.
6. Bonus one: try opening the protective cover everytime you charge/sync the phone with your nails trimmed!
The Galaxy S5 is water- and dust-protected, and that's great, but it also means that you have to open and close the cover that protects its microUSB port every single time you want to charge or sync it, and that's mighty annoying.