^From where I stand the only thing LotSB had that Leviathan lacks is a significant character arc. Of course that's not the point of the DLC, it's a mystery/exploration story so I'd say it's at least equal to SB in terms of quality and content. Anything else is just personal preference.
What really brings it to life for me though is the inclusion of squad interaction. It's something that always made the ME2 DLCs feel just a little...off. Now that I think about it though, it was really only noticeable in Overlord & Zaead's mission,In LotSB you had Vasir and then Liara chatting with you the whole time while 'Stolen Memories' 'Arrival' dodged the issue entierly by having you play solo.
Still, if this is a sign of things to come I think we're in for a treat with future DLC.
Oh and apparantly the data miners have already turned up something. A new level called "CitHot" IIRC. That's in addition to the "OmgHub" levels that showed up in the firefight pack. Apparently they're certain it's nothing to do with Bryson's lab, so speculation ranges from a hotel to the keeper tunnels.
Personally, I think if the trend of giving the Catalyst a few more lines to say with each DLC holds up, then one of the future stories should deal with the race that first designed the crucible. I mean why else have him say "you would not know them and it'd take to long to explain"
if there wasn't a story behind that. They could easily have had him say something like "they were called the Zoidbergs and they took longer than most to finally eliminate."
My pet theory is still that the keepers were behind it. Who else knows the citadel better? Who else has the enormous technical expertise to conceive of it? And who else has more of a reason than anyone to want the endless cycles to stop?