I'm VERY happy to see this development!
Dungeon pannel has a new system where you use a keystone to "up" the difficulty of the dungeon, and apparently you can keep increasing that level of the stone for increased rewards in the dungeon. Gone is the rush challenge mode, but there IS a basic timer to meet - but the key is to just beat the dungeon, not rush through - so CCs and what-not are very important.
Also there's a new variable system that adds variations to the dungeons. Things like Every player takes slow Damage over time, all NPCs enrage below 30%, ect that can be applied to your dungeon to make it more difficult (and greater rewards).
They definetly say they get hard later on. even using the words to something of the effect of "You will find your own personal breakpoint in these that will break you".
Big news from Blizzard's own mouth is...
DUNGEONS CAN COMPLEMENT OR EVEN REPLACE THE NEED FOR RAIDING!!!
EDIT: In other dungeon news - there's 10 dungeons in Legion. 5 of which are leveling dungeons, but - like the overworld - can be done/scailed in ANY ORDER/ANY LEVEL and you will still get the right level scale/xp for your particular level!