Connection issues.
Already climbing.
Not just connection issues although those are probably the most obvious, there's also compatability issues so some PC's can't run the game without changing the exe launch settings to run in Windows 8 Compatability mode and granting admin rights (sadly not everyone on PC are tech savvy enough to even do this basic leg work, not that they should have to)
There's also the fact it uses Kernal 0 anti cheat, which is ALWAYS unpopular, especially the one they went for, its quite literally the worst anti cheat on the market, coincidentally its also the cheapest which is why its normally only found on terrible dated unpopular Korean MMO's.
But I suspect it will climb when a majority of these are sorted.