No doubt there will be nerfing and balancing done but I think the concept behind the DK is so strong that you'll still be tackling elites with relative ease.
They shine very well in a vacuum of solo grinding/questing - perhaps more than any other class. I'm astounded how well they tackle encounters and there is very little downtime (if any at all).
Their performance in a raid is on-par with well geared classes.
I've seen 'em roll in heroics, 10-man, and 25-man settings… not once did they top the damage meters. Hunters and Mages still own that spot, followed by Warlocks and Shadow Priests (at least on AoE trash).
They make fine AoE tanks, that much is for damn sure. I also like how a Priest's Devouring Plague counts as a disease on the target for them to spread using Contagion.
There's currently problems gearing up a DK knight due to itemization but recent and incoming patches plan on alleviating this somewhat.
I'm nearly 70 with mine and thus far it's two thumbs up. Fun class.