Edit 2*
I have now writted a better guide on how too fix the issue. http://forum.nostalrius.org/viewtopic.php?f=45&t=37735
---------------------------------------------------------------------------------------------------------------------------------------------
Hello! I just managed too fix an issue i have seen multiple ppl on this forum have, but none had figured out how too fix.
I was not able too select my monitors native resolution (1920x1080), but only had 4 options (the max i could go was 1620x1200).
A lot of ppl suggested going into the Config file and change the resolution in there, that did not work. Some ppl suggested that you start up retail wow, close it and restart the 1.2.1 client. When i did start up my 1.2.1 client it said "New hardware changes has been made" (Or smthing along thos lines), and it asked if you wanted too reload settings, i tried both yes and no, none helped me.
The fix too my issue was that my integrated GPU (on the motherboard itself) was being detected, and while all new games had no issue using my acctualy GPU the old wow client could not do this.
Too fix this issue i had too go into my BIOS settings and disable my integrated gpu usage for multi monotoring.
Sorry for bad english, hope this can help some poor souls with the same issue i had
Edit
My setup while experiencing this isse
Motherboard: MSI Z97
GPU: GTX-970
CPU: Intel I7-4770hk
Monitors: BenQ GL2440H (Digital) + BenQ GL2450
OS: Windows 10
GPU Driver(Installed thro GeForceExperience): 364.51
Ram: 16 gb - 8x2 (Dont know brand)
All other drivers i had where installed thro Windows update.