I noticed a long time ago that there isn't much difference between the sound of country music and rock music nowadays. If I'm surfing my radio and I come to a station playing jazz, classical, or rap, I recognize it right away; but if I come to a country or rock station, I often can't tell which it is until the song is half over. Country songs often have screaming guitars that would fit equally well in a hard rock song, and only a minority have a recognizably country instrument such as a fiddle or a banjo.
All the same, no one could listen to a country music station for more than a few minutes without realizing that it is definitely not a rock station. What are the differences?
There are some stylistic differences. Country songs are more likely to have a boogey beat or certain western-sounding bass lines. They also tend to be more formulaic: if you can't predict how a country song ends, you probably don't listen to much country music. But those things aren't enough to identify them distinctly.
What really sets country songs apart, I have realized, is the content, i.e., the lyrics. For one thing, there are few country songs in which you can't make out the lyrics, whereas rock songs are famous for having lyrics screamed incomprehensibly or drowned out by the music. I can't think of any cases of country songs which, like "Born in the USA," are often misinterpreted to mean something entirely different from what the singer intended. I think this is because lyrics are more important in country music: you also find very few songs with crypic lyrics or outright gibberish, such as the ones in "Bohemian Rhapsody" or "Smells Like Teen Spirit."
Most country songs, like most songs of any secular genre, are about love. But country songs are more often about faithfulness in marriage than rock. I mean, not just songs complaining about an unfaithful spouse or boyfriend/girlfriend, but songs in which the singer successfully defeats temptation (such as Randy Travis's On The Other Hand). Country singers are also more likely to express gratefulness about their position in life: their spouses, their children, the town they live in and the country they live in (Montgomery Gentry's I'm a Lucky Man). How many rock songs mention the United States other than to criticize it? There is hardly any country song of this variety; even when they complain about problems in the U.S., singers almost always express gratitude for living here rather than somewhere else. The gratitude theme extends into other areas: I believe country songs are almost unique in praising a life of hard work and raising a family. They don't sugar-coat it, for the most part, but they do hold it up as a worthy goal (e.g. Just Another Day in Paradise). Finally, country songs are far more likely to be overtly religions (strictly Christian, as far as I can tell), but, far from being sentimental schlock as the culture mavens would have you believe, they are often filled with honest scepticism as well as sincere faith (Lonestar's I Pray or Brooks and Dunn's Believe).
The academic in me wants to pile up more citations to prove my case, but I'm not trying to prove it, really; I just want to point it out, and give some examples to show what I'm talking about. The point is that country music commonly praises what used to be quaintly known as "virtues," and this, rather than strictly musical qualities, separate it most distinctly from rock.