The first game looked nice and you wanted it to be half decent but it was a pretty lame game. Does this improve the game enough to make it replayable (its main fault - one game you're meh)?
The expansion adds what I think FFG felt was missing from the game when they acquired it from Dust.
You have the ability to Wound thieves now. Earn Experience Tokens which can be spent to re-roll a die or buy equipment (you earn one each time you roll a 6 when attacking in combat). There are new discoveries added to the game (things like explosive traps and hidden passages). The Merc's have gotten overhauled. Other rules improvements were made (such as moving through your own thieves).
The catacombs are very risky and add a sort of euro feel, with the risks you take to get the good treasure.
Honestly, I wasn't a fan of the base game either. Not at all. It felt too shallow when played with less than 3 players. This expansions additions really fixed the game and improve its fun factor.