I think there's a lot about Layla that makes her stand out from your standard WWE Diva.
Pretty much all the Divas look hot and are the type of woman that most men would love to have a sexual relationship with. Layla is no exception but Layla is someone that's got a lot of heart and honestly does put her best effort out there. No matter what she's been asked to do, Layla has always tried her best to get it over even if it outright sucked. I have to give her credit and respect for that. She doesn't take to Twitter or Facebook to piss and moan about how she's not being "used right", she just does her best like a real pro is supposed to. I think she also showed a lot of heart coming back from her knee injury. She tore both her ACL & MCL that kept her out for almost a year. The fact that she's still pretty good inside the ring is an indicator that she worked hard to get back into ring shape. I can't help but wonder how many Divas would just call it quits and hang it up after sustaining such an injury. Layla strikes me as a woman that genuinely loves wrestling and being a part of it. She's not just there to look pretty in revealing outfits and making a relatively easy paycheck. Layla is also more than just a pretty face. She's got genuine personality and can do well as a face or a heel.
As for why she's Divas Champion, like a few others have mentioned, there's just nothing going on with the Divas right now. Layla's been out for a while and it's been a while since WWE had a strong female as Divas Champion. There are several women that have the necessary talent in WWE, such as Beth Phoenix or Natalya, but they've never really clicked with the audience. I think they could if fans are given a reason to care about them. Also, right now, there are simply much bigger & more relevant things going on than the Divas: the MITB ppv is coming up as is SummerSlam, the 1,000th episode of Raw happens in two weeks, Raw is going to 3 hours, etc. Plus, Kharma has been out for over a year, Kelly Kelly is taking time off and might not even come back, AJ is off doing her own thing right now as part of the Punk vs. Bryan feud. Right now, Layla is pretty much the best choice I think.