There are no serious faults to my knowledge. There are a few minor ones.
- The camera has flickering black likes when you turn on WiFi. (Similar to the ones you get on any camera when you point it at a screen or fluorescent lamp)
- The camera also tends to be highly, highly rubbish in low light at ISO Auto. Not a fault per se, just a result of the tiny sensor.
- On 4.4.2, some people have trouble with WiFi passwords not being saved. Still investigating that (people are not being very helpful) but it seems to be auto0backup or sync related.
- 4.4.2 has the irritating SDcard permissions. If you're going to root, not a problem. If you're not, some apps might not be able to write to it, depending on wether the developer updated the permissions.
- The homebutton is 'crooked', meaning it's a bit wibbly. Very, very slightly. Nowhere near as bad as the powerbutton on the S3, though. (You could wiggle that about to the point where it fell out or got stuck and turned off your phone.)
- Knox can be counted as a problem if you're outside the EU. (If you're inside the EU, your hardware warranty is still valid with 0x1.)
- Some people complain about GPS issues. I've personally not encountered them. But then I don't think there's a device where people don't complain about the GPS.
- There's also the Android System, Android OS and Mediaserver battery drain that affects every kitkat device (including the Nexus and S line.)
- Some people have complained about there being a small gap between the screen and the metallic(plastic) rim. Mine has that too, but it's not fatal, just annoying if you're a bit OCD. (Which, let's face it, we all are.)
- People have also complained about the metallic coating on the rim eroding quickly. That's related to the PH value of their skin and/or the case they're using.
- The Spen may need to be calibrated. Pop off the plastic button carefully (dn't break it!) with a small screwdriver (it jumps, careful not to lose it!) and twist the potentiometer closest to the pentip to increase or decrease the sensitivity. Or give it a good whack on a table, that often works too. (These are both fixes that apply for most Wacom pens. As an Intuos user, I've had to do the potentiometer tweaking fairly often.)
- The multitouch problems were mostly fixed in a few OTAs.
- There've been complaints about the speaker quality as well. But then, you can't expect Bose quality from a 1x0.2cm speaker in a plastic box. That's what they invented portable speakers for.
That's about it, I think.