#4 Sin Cara

In the quest to find the next Rey Mysterio, WWE signed Alberto Del Rio and the original Sin Cara with the hopes that either man would take over as the next Latino star in WWE.
Well, they failed on both fronts. Both men who put on the mask were prone to a botch here and there and without speaking, failed to really connect with the WWE Universe.
Did you know a top WWE Superstar just wrestled outside the company? More details HERE
Those two factors have kept Sin Cara from being little more than a opponent for people to beat in warm-up matches.
He did have a great match with Andrade when the latter debuted last year, but he got hurt and has been nearing a return to action for some time.
Since the roster is already stacked from top to bottom and he's already had runs in a tag team (Lucha Dragons) and as a singles' competitor, there's little more for him to do than what he used to do.
A release of Sin Cara wouldn't really put a dent into WWE as there are several more talented stars in NXT and yet to be signed.