Opinion so far:
Swamp / Jungle:
You need to be coordinated to get the wisps into their trees. The Mossman who appears and disappears is great too, keeps you on edge and is a neat little gimmick. Boss was cake. Visually, a very interesting place.
Aquatic Ruins:
Done it twice. Once was the darkness path that seemed incredibly tedious, but we tried to rush it and didn't really figure out what was going on until it was too late. Luckily, one of us made it and got us to the next checkpoint.
Second time, we took the Dolphin route and that was a blast. Very satisfying, though very easy to get killed. A lot of us finished the Cycle missing some armor, so it might be a good idea to strip that off before you attempt this part.
Overall, quite easy. Feels like a breather between some of the others.
Volcanic Fractal:
Has some nice, light jumping elements, the bosses were fun. Very Indiana Jones-ish.
Underground Dredge Facility:
VERY fun, reminds me of Phantasy Star Online's challenge mode (people splitting up to provide a path for the rest of the group to advance). Unfortunately, our fifth didn't have mumble and disconnected during the boss, so trying to get the strategy to work just didn't mesh and we ended up bailing. Still a BLAST though.
Snowblind:
Very cool; the rushing between bonfires is neat, the elemental boss was challenging but fun, the dash through the darkened forest was very cool looking, the last boss was pretty by-the-books though. Not bad at all though.
Urban Battleground:
Insanely hard, even on level 1. I dunno if we were just missing something or what, but we were getting torn apart by siege and constant spawning vets, and getting near the door is rough. Once you clear that first gate though, it's not bad and actually pretty fun. I think we missed something on that first part though, once that part is down it will be a blast.
Haven't done any others. But damn, these are GREAT, and nice and short too. Can see this being my favorite MMO feature ever.