LTE connectivity. More exactly, the OnePlus X supports the following LTE bands: 1, 2, 4, 5, 7, and 8. This doesn’t completely cover the LTE bands of AT&T (2, 4, 17) and T-Mobile (2, 4, 12) – these being the only major US carriers that you can use the OnePlus X on, since we’re talking about a GSM device here.
According to Droid-life, OnePlus does not plan to add support for additional LTE bands on its smartphone, so you won’t be getting complete LTE coverage. In fact, AT&T’s and T-Mobile’s LTE networks heavily rely on bands 17 and 12, so you’ll likely be without LTE coverage quite often.
LTE issues aside, if you’re willing to buy the OnePlus X in the US (where it’s going to cost $249 unlocked), you’ll need an invite (at least in the first month of availability).