I have found that there are certain things, even in supposed "gateway" or beginner games, that scare off my group of friends. One of them is unfamiliar game mechanics. The way that many designer games work is counter-intuitive to someone who grew up on basic American games like Life or Uno. For instance, many of the card games like Bang! or Fluxx require a player to draw a card first and then make a play. This trips my friends up every time because they are accustomed to playing games where drawing cards is a penalty.
Another thing that puts my husband off are games where most of the instruction is on the cards. Before I started researching games, his favorite card games were Uno and Spades. In those games, the gameplay is pretty much the same for every player. I thought he would like the newer card games because he likes games with a bit of a "take that" aspect. How wrong I was! He found Bang! to be confusing, and Give Me the Brain! to be chaotic. The gameplay didn't move fast enough for him because everyone had to read their cards to figure out what to do next.
Settlers of Catan or Ticket to Ride are usually mentioned as good gateway games, the sort of games that will draw people into the hobby and prepare them to play more complex designer games. Just from reading the rules I can tell that these games would only frustrate my group of friends. If you are like me and don't have an experienced player to present games to your group, then you should look for a game that has only one unfamiliar mechanic. That way, you won't overwhelm your friends and they will be more open the next time you show up with a new game.