The worst "feature" offender I have found is that when talking on the phone, if a call comes in, the screen activates. If the phone is close touching my ear, it has a habit of picking up the incoming call without any notification.
It fails miserably on the second one. The only options to import contacts are through Facebook, which I don't use, and from a SIM card, which is what I ended up using. Pretty limited, to say the least, but the real kicker is that it has no option to actually export contacts. So if you use it for a while and decide either to wipe the device, or to stop using it, you have no option to retrieve contacts you've since added. I've investigated if it was possible to at least back them up using an ADB shell (it is, supposedly it's an sqlite DB), but apparently you need root on the device to do this, which isn't possible on the default install. I didn't investigate any further when I realized I was actually using Android debug tools at 3 am to get my contacts out of a supposedly consumer ready device.
Needless to say any software updates probably aren't forthcoming from ZTE, since even the upgrade routine fails with an error on these devices (https://support.mozilla.org/en-US/questions/967817).
Another thing that bugs me is that while Mozilla are distancing themselves, and their branding, from the likes of the Geeksphone Peak+ (http://techcrunch.com/2013/07/25/peak-plus), which as far as I can tell receives timely updates and has a vibrant community, saying it's not a Firefox OS phone, they're also busy directing people to the manufacturers (ZTE in this case) whenever there is a problem with the officially blessed Firefox OS version running on the first consumer grade FFOS phone.
Next time I waste more time trying to get basic functionality working, I'll just build my own updated images of FFOS (https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS/...) and see if things have improved by then.
I can understand some of the missing features (such as can't upload files, no clipboard, the keyboard is small and irresponsible or only en_US available), but some bugs are really painful, being the biggest one when you're browsing a website and the the browser hangs leaving your mobile completely dead for way too long (I had to remove the battery a couple of times). For an OS that is basically a browser, this is really bad.
Also I've found both the wireless and the data connection to be unreliable. Sometimes it refuses to work and only a reboot will fix it.
I'm not an advanced Android user, but I can't do the few things I used to do before using the ZTE Open as daily smartphone. As I said the ZTE Open needs an OS update, really.
I've had problems with the UI not responding to touches (never on the keyboard, so I'm pretty sure it's a software issue), and with data cutting out, and not coming back until I toggle data several times.
Admittedly, I'm not sure whether these issues are hardware, the particular beta software I have, or FFOS (I should have been filing bug reports).
1. Browser only loads one tab at a time.
2. Inability to lock the screen orientation to either Portrait or Landscape.
Issues with ZTE Open:
1. Mediocre touchscreen sensitivity.
2. Short USB cable.