There's no question that fans are most worried about the back four of the U.S. before the World Cup.
Omar Gonzalez and Matt Besler will likely be the starting center-backs against Mexico. They've proved to be a good pairing against CONCACAF competition, even shutting out Mexico in the Azteca last year, but it's hard to have confidence that they'll be rock solid in the World Cup against the likes of Portugal and Germany.
Individually, they've been great for their MLS clubs. Gonzalez was the MLS defender of the year in 2011, while Besler earned the same honor in 2012. However, on the national team level, the big question is whether or not they've developed a strong enough chemistry to go up against some of the best players in the world. Unfortunately, it seems as though that question won't be answered until this summer.
The full-back situation for the U.S. is an even bigger question mark. With Brad Evans injured and DaMarcus Beasley denied release from Puebla, Klinsmann isn't left with many options at full-back for the friendly against Mexico.
DeAndre Yedlin, Tony Beltran and Michael Parkhurst are the only natural full-backs on the roster. It seems likely that Yedlin will get the start on the right with Parkhurst on the left. Another option would be to experiment and try out a wide midfielder like Brad Davis at left-back.
When the World Cup rolls around, Klinsmann will have a big choice to make. Fabian Johnson looks to be the best option on the left side, but he has also been one of the better left midfielders that the U.S. has.
On the right side, we could see Geoff Cameron, whose experience as a right-back in the EPL could prove to be a valuable asset going up against players like Cristiano Ronaldo in the World Cup. Or perhaps Klinsmann will like what he sees from Yedlin or Parkhurst against Mexico and consider them for a starting role.
With all these options, it's safe to say that full-back is the position that remains most unclear leading up to the World Cup.