I ran two wings of Altdorf Sewers last night with some guildies. The first wing (on your left as you exit from the flight master) was pretty simple tank and spank fare, level 14 or so mobs. Also pretty short.
The second one...well, Mythic really doesn't ease you into shit. Even though I was level 22 and most of the mobs were 18-20, there were some pulls with ridiculous directional AOE damage, groups with a "master" that would randomly tell the other mobs in the group to attack a single target (and you can't taunt off them), bosses that do the same thing but don't announce who they're sending them to attack to, a dual boss fight and both drop little AOE runes on the ground that continuously tick for 100 damage (we didn't realize this until about the second time we fought it, having 8+ of them around your group will end things pretty quick), other pulls that spawns new mobs at a fairly fast rate and the spawns have verrrry high HP, and an end boss that we sorta exploited to beat on accident (you're supposed to fight him in his room and he spawns waves of critters. We didn't realize this until after we beat him and went into the room to check it out and like 50 of the things just ripped one of my guildies apart).
It's not stuff that takes too much to figure out if you're an experienced MMO player, but it's a really big step up in encounter complexity for only the second Order instance.