The Europe map is more complex and interesting, plus the game offers the addition of train stations, which are a neat tool. Marklin is good, too, but adds too much with the passengers, IMO.

I would add that you should definitely pick up a version of TTR for your collection. It doesn't necessarily offer new complexity, but it is such an entertaining system with long-term playability.

Dominion, as others have said, is a no-brainer. Puerto Rico and Race for the Galaxy never grabbed me, though. That's the thing: with this hobby, you have to be prepared to throw down $40 and end up disappointed sometimes. Games that seem like sure bets because they're in the top 50 on Board Game Geek aren't guaranteed entertainment for everyone.

I would also recommend Stone Age, based on our shared appreciation for Settlers and Carcassone. It's a step up in depth, but not too far.