Del Rio is nowhere near a babyface potential. Unless you consider the possibility of him being the Eddie Guerrero 2.0 which is never going to happen in today's WWE. He's got the face, the accent, the attitude, a sidekick, everything to make him a perfect textbook heel with a punchable face. There is just no way he could work that style. He's a natural heel, just like JBL was (who BTW is awesome as a commentator).
Sheamus really isn't over with me. I think I'm that part of the audience that doesn't cheer him, because he's built up ridiculously strong and has not had great feuds.
Everyone wants to turn heel, because it's way easier. Guys just don't want to run around kissing kids, hugging people or dancing in the ring like a prat. Faces right now who are mostly used:
Cena, Ryback, Sheamus, Orton, Mysterio, Cara, Kofi, Santino, Brodus Clay, The Usos, Christian, Justin Gabriel, Primo & Epico, R-Truth, Ted DiBiase, Tyson Kidd, Zack Ryder
Highlighted those that are decent to good at their act and are likely to remain faces in the long term. Some of them like Kidd, Gabriel, Primo&Epico, Ted DiBiase, The Usos, Christian are victims of the writers giving them useless gimmicks and storylines. They'd be much better if they got a decent push, but no..
On the other hand you have men like Orton, Truth or Christian that are just miles better as heels. Then there are the joke acts of Santino, Clay and mostly Ryder and aren't booked well at all.
Punk, Daniel Bryan, Kane, Big Show, Alberto Del Rio, Cesaro, Rhodes&Sandow, Young&O'Neil, Otunga, Ziggler, McIntyre&Slater&Mahal, Swagger, JTG, The Miz, Tensai, Barrett
In green are the guys who could pull off being a face IMO. Some even have a chance of being turned face like Bryan who is unbelievably over, Rhodes who is just very very charismatic, McIntyre who is essentially a copy of Orton but he's more natural in his expressions (had at the time the best entrance song "Broken Dreams"), Swagger is probably going to come back as an All-American face who could be actually very over if done well and there is The Most AWESOME star in Miz who's got almost everything to make a faceturn good for him. He's still great as a heel. Easily the most punchable face in the WWE.
The rest of the heels I think should stay as heels in the long term. Some also suffer from the gimmick and shitty writing e.g. Kane, Del Rio, Cesaro, Tensai who can be so much better.