A great dive bar has to strike a delicate balance between grungy and comfortable. It should feel like the cozy little hole in the wall where the floor isn't that clean and the booths are slightly broken, but the beer and liquor are well stocked and the characters are colorful. Think dirty enough that T.G.I. Friday's managers would be disgusted, but safe enough that you're not getting a knife in your side when you sidle up to the bar.
And in Las Vegas, Huntridge Tavern is that bar. I head here a lot during happy hour and find a collection of folks who are happy to be drinking--the clientele is usually a mix of folks just off work, folks down on their luck and smart ass regulars lined up along the bar talking to the bartender. There's a lot of attitude is this bar. Go with it and you'll have a good time.
And Huntridge follows through on the crucial requirement for a dive bar: cheap drinks. Beer is $2-3 from the tap, liquor was pretty damn cheap (for a generous pour as well). It's definitely my favorite hole in the wall in town.