So, I've never seen this Sin Cara guy before, but the commentators definitely seemed to be overselling him on Smackdown. They kept saying stuff like "you're in for a huge treat" etc, and he just struck me as a poor man's Rey Mysterio. Is there something more to him than was shown in his return match?
Sin Cara was formerly known as Mistico, he was a huge star in the CMLL promotion in Mexico (a Mexican John Cena if you like) and was also a big draw when he competed in Japan. WWE jumped at the chance to sign him because they finally woke up and realised that Rey Mysterio's career as an active performer is nearing an end (he's had 6 knee surgeries...). Furthermore, Mysterio has had some conflict in the last couple of years with WWE, to the point where he has actually asked for his release.
WWE panicked because Mysterio is a huge draw for them and if Rey were to retire or quit, then they could lose a lot of business they built up in the Mexican market as a result of Mysterio's success. WWE decided that they were going to find a successor, so they looked for a huge drawing international star that wore a mask and wrestled a fast paced, action-packed style - they found that in Mistico...
...the trouble was that Mistico/Sin Cara was brought straight up onto the roster and wasn't given a spell in FCW to learn how to adapt his style to the American style of wrestling (Mexican wrestlers work the right side of the body, whilst American wrestlers work the left). Following Sin Cara's debut and realising that they had royally screwed up, WWE quickly put Cara into a programme with Chavo Guerrero (who is an experienced luchadore) so Cara could work his native style - but - the botches continued and Cara was then suspended for his wellness policy violation. It seemed that WWE was going to end it there and then and ship Cara to FCW - or perhaps even release him... but, this was Triple H's first signing as talent exec and therefore it had to be a success. So WWE called up Hunico (a long time rival of Mistico's in CMLL) from FCW and put them in a programme together that quickly ended at a Raw taping in Mexico (so they could salvage at least one big draw out of it).
Sin Cara suffered a knee injury last year when he landed badly after a dive outside the ring and along with the mask, that's about all he and Rey Mysterio have in common. The only reason Cara has been given so much space is because giving up on him would make Triple H look bad. I think he will eventually come into his own, but only if WWE handles things correctly.