1. It's in the Cosmopolitan. That's worth at least 3 stars to begin with! Seriously, if enough people haven't said it yet, the Cosmo is the place to go to feel hip in Vegas. Maybe in the entire United States. It's so cool here, I'm amazed I'm even allowed to hang out.
2. The ambiance. The bar atmosphere is just slightly different from the atmosphere in any of the other casino bars or the casino itself. It's very open to people walking by, but kind of private, with comfy chairs/couches to lounge on. It's still super cool, of course.
3. The drinks. They are delicious, especially the oft-mentioned hot mule. Seriously, if you care about cocktails at all, you need to try it.
The only reason I'm not giving it 5 stars is because of the prices (~$14 a drink, which I realize is average for the Strip) and because its hip location actually works against it. I mean, it's tough for Vesper to stand out TOO much, when the Chandelier, Book & Stage, and Bond are actually in the same building.