Different from WoW; they reuse the dungeon setting but will usually dramatically alter the map and change all the mobs to continue the story. They're not harder than the originals, and I think are probably uniformly easier.
Leveling dungeons tend to be harder and longer, for the most part, than the endgame ones. The level 50 dungeons and again at 60 are basically designed to be completed in sub-20-minutes-each and are just massive AOE fests. Leveling dungeons are longer and generally slightly more difficult because they're not prone to being out geared like the at-cap dungeons are. The daily tome dungeons can be completed in ~12 minutes with a decent group, and 20 with the average mix your get in DF. Leveling ones are longer but generally require more thought.
The general pattern in ARR and Heavensward is to have one brand new dungeon per content patch, and then 2 (in ARR) or 1 (in HW) 'remix' dungeon; the Expert roulette consists of the dungeons from the most recent patch, so in HW it's been two dungeons it roulettes between daily.
Hm interesting.
Now that I'm saying it, the version WoW has now with the dungeons getting harder and harder until you die is really cool, I hope they add that in Stormblood, since Heroic dungeons in WoW were very similar once everyone gets geared and breezes through them.