From another forum:

Quote Originally Posted by Him
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I'm eagerly getting ready for my first dungeon soon.
Quote Originally Posted by Me
Ascalonian Catacombs story mode [L30] was tough, but fun. Of the three that I've run, this one was the hardest, which I find strange as it's the first dungeon available. Explorable mode [L35] was ROCK HARD. Punitively so in certain parts.
Caudecus Manor story mode [L40] was fairly easy, and really fast and short in comparison.
Twilight Arbor story mode [L50] is what we just ran a moment ago. Four guildies and a fairly competent pugger. Difficulty between AC and CM, and my favorite dungeon so far. Tough as nails until you figure out what is happening, and why, and how, and when. And then after you figure that out it's fun without being quite so tough. Just the way that a story mode dungeon should be in my opinion. I really think they nailed TA for story mode.