Really not a nightmare at least in 8m 16m is apparently a bit rougher but still not worthy of nightmare mode.
Seriously, try NiM tanks and tell me how close you get. You'll learn things about pushing your guild to the limit that you never thought you'd need to do as far as specifics. As far as how bioware is doing, I think the most we can ask for is steps in the right direction. The fact that we'll, as a community, most likely make it to the 3rd week of progression with bosses still alive is a credit to them. I think the biggest question now is less about how difficult NiM EC is and more about when NiM TFB and/or operation #5 is coming out. That will dictate how many people/guilds keep going (as far as ops at least).