You're right Pleph; music theory isn't an absolute science. Since what makes music good is ultimately a matter of personal taste, there's as much potential variation in what will be "correct" as there are people in the world. Luckily for music theorists most people are acclimated to the musical idiom they have been exposed to all their life, so there's enough agreement over what's "good" that lots of general observations which will apply to almost all western music can be made. And there you have music theory.
I can understand how you feel; I also like well grounded absolutes. It's a great feeling not only to know that something works in a certain way, but to know why as well. Luckily for the curious there's also a lot of absolute knowledge relating to music to be gained. It's generally low level, though, and not always directly applicable to music. Music theory provides a helpful intermediary step which can often help us write and understand compositions.
To answer your question on the scales: there's really no universal implication, as such. Major scales are appealing to you because you are used to them. There's no jarring there; there's no strangeness. Their appeal lies mainly in their familiarity. I won't say that different scales don't convey different feelings, but the feeling of a major scale is one you are accustomed to, and therefore comfortable with. You (and most of western society) are also accustomed to harmony constructed in the "usual" way. That's means that songs stick to a key and harmonies obey all the "rules" codified in the large body of music theory. Mostly.
So, when you write song in C major the only real implication is that if you use notes from the C major scale and stick to traditional chord progressions, then you'll be very likely to produce a song whose sound is easily acceptable to most listeners. That doesn't invalidate other conceptions of music, or make western music theory a universal description of "proper" music. It's the difference between giving the average American a cheeseburger and giving him baba ganoush. So yes, if you were to use lots of out of key notes, then it wouldn't sound right, where rightness is judged by adherence to accepted style. Actually you might end up sounding jazzy, which illustrates the way that musical taste will adapt to unconventional style. Jazz harmony was strange at one time, but not so much any more.
I've said a lot here.
I've talked a lot about how music theory only describes how the music which people have grown to like works. That's because I've had a bee in my bonnet lately about mindless adherence to the accepted norms of music composition. I'm not saying that those norms are bad, or that you shouldn't compose according to them. I just want to stress the fact that the "rules" only describe what people are used to, and not some kind of absolute. Know that your options are wider than all the music you've heard combined.
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