Miss Vienna
I have both of their albums and their Christmas album as well. They are in fact one of my favorite Country bands, right up there with Sugarland and Little Big Town.
I think that so many people find them pleasing outside the Country Music circle because they are fairly unique. While their sound is distinctly country, they borrow a lot from Pop music (much like the Princess herself, Taylor Swift) which helps a larger audience relate musically. I also believe that they have a distinctive male-lead
and a female-lead voice type that they use interchangingly and often blend depending on what song you listen to. It's different than most country music artists and groups and people like that.
You explained what I was thinking exactly...
I like Lady Antebellum but at times I get a bit sick of listening to them and have to take a break.