She cried for the same reason that the Recon Squad feels down about - Operation Failed means that the sacrifices were for nothing. You can somehow justify the body count if you got results, but to fail means that they were wasted more or less. She failed in her objective to capture Eren which she perceived was for the greater good, while the Recon Corps failed to capture the female titan which they also thought was for the greater good.
It's important to work out what you have to do, and focus on accomplishing that. When you become greedy and yearn for more, you risk losing what is really important to you. Levi showed that in how he acted. Mikasa almost died while trying to kill the titan when it wasn't necessary. The body-retrieving pair demonstrated right afterwards that when such greedy actions backfire, it can be catastrophic.Originally Posted by MFauli
Small, steady steps as they say.
I feel that Armin has the potential to become someone like Erwin. He's smart and had a vision for venturing outside the walls. The thing that was holding him back was his physical weakness and his lack of balls (that latter part comes with being smart and being able to perceive risks). With this expedition, it seems he's capable of going through with the best possible actions and hardening his will. He currently lacks charisma, but that will come when he hardens up. It's not like Pixis was the most appealing person, and it seems Erwin is so elusive to most people that they wouldn't really follow him for his sake anyway.
Usual anime trend would put Eren as team leader though due to his passion, with Mikasa following him with great skill and even greater devotion, leaving someone like Armin to deal with strategy and Jean to lead the city defence/administration.