The expansions looked interesting, and I liked the fact that some of them add extra tile space. It's a trade-off though... More tiles = more content, etc, but also longer games. I enjoy the fact that you can play more than one game per evening. (as opposed to, say, Risk) I also enjoy that it has more to do with strategy than raw chance, where Risk can leave me completely frustrated. (1vs100, still winning, etc) Sure, you're still at the mercy of each player's die toss, but it's not deciding any combat.

Being able to trade resources with players adds another layer, too, since it depends on your persuasive skills. Whether or not that person knows you well, or if there's a mutual threat you're both working against, can drastically change your resource situation. You can use other players' progress to make trades look like a good deal, while it may actually help you win the game a few rounds later.