There are a couple of very good answers already, and I think that Bob Black's answer may be the best, but I would like to add to that with regard to how some people might go about classifying music according to a Christian morality.
Commandments and principles versus conviction
To begin, it is important to distinguish the difference between a commandment, a principle, and a conviction. A commandment is something that we're giving directly from the Bible. It's a "thou shalt" or "thou shalt not" that you can find somewhere specific in the Bible. Then, there are principles which are themes that develop in the Bible and which can guide us on certain matters even though they may not be spelled out directly. Finally, there are convictions. Convictions are different because not only are they not spelled out, but they are dependent upon things like culture which change. They should always be informed by principles and commandments, but they should never be mistaken for them. These are important distinctions, especially with something as subjective as music.
Because this conversation is one which relies more heavily upon convictions than upon commandments or principles, the divisions may involve Christians cultures, that is to say groups of Christians who share similar age ranges, localities, political ideologies, etc., rather than being divided strictly along denominational and doctrinal lines. You may find that, for instance, even within the same local congregation you can have groups which believe doctrinally the same but disagree on many of these points. Consider that a person living through the 60s or 70s may may still recall how long hair (even beards) and music with distortion was associated with a large movement actively attempting to subvert Christian morality, but to somebody in their early 20s today that same thing may seem old, tame, or even harmlessly energetic from their reference, and so the implications are different.
Verses to consider
The Bible does say a lot about music, but most of it is about good music with little being said about bad music directly. There are passages that seem to approach it, but they rules must still be implied. However, we do know some things that are generally good for life:
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.
We can see that our thought life is important, and that we should limit our thoughts to pure things. Yes, there is bad in this real world and we must live through it. However, when we direct our thoughts, we should strive to think on the lovely things.
2 Corinthians 20:5
Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;
Music can often bring thoughts and feelings into our heads and hearts which can be very hard to dispel once they are there. Perhaps everybody doesn't have the same temptations, but I believe that there are a few who have had to adjust their musical library because some songs would bring such thoughts that were hard to bring into captivity.
As Bob mentioned, there is 1 Corinthians 6:12 to consider, as well as:
Hebrews 12:1 (emphasis mine)
Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us,
We are in a race. The purpose of our time here is to seek God with all that is within us and to spread the Gospel. We are in a war, or as Paul pictured it, a race, and in such places there is little time for things that can distract us, even those things which might otherwise be good.
Finally, consider 1 Corinthians 8:
1 Corinthans 8:12-13
But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ. Wherefore, if meat make my brother to offend, I will eat no flesh while the world standeth, lest I make my brother to offend.
That chapter presents a principle which teaches us that we should watch out for the conscience of other weaker believers. Even though the specific reference here is to meat offered to idols, the principle is made clear that it is possible, in the exercise of our own personal Christian liberty, we may cause a weaker Christian to stumble and that doing so would be a sin.
These provide a sense of the larger principles principles which have informed many Christian cultures to form their views on music.
The message, the music, and the man
When looking at morality in music, I believe that it can be broken down into three areas:
- The message conveyed directly by the lyrics.
- The musical structure itself.
- The artist performing the music.
Different Christian subcultures will view each of these differently, typically with the most contemporary considering only the first point and only the most traditional seriously considering all three.
The first point is the most obvious. If there are immoral things in the lyrics directly, then the music is immoral. This may differ to some degree with the particular denomination and their views as to what is explicitly sinful, but I am unaware of any denomination that wouldn't have some trouble singing along to lyrics which openly denounce Christ or worship the devil directly (and there are such songs out there). Therefore, this is where much of the debate is centered. The most fundamental groups would prefer, at least for worship music, that the lyrics are clear, strongly doctrinal, and which teach the listener some truth about Christ and the Gospel.
The second point isn't considered by all Christian cultures as relevant, but to those who consider it, the discussion seems to break down into the effects of the music upon the body and mood as well as the perception of order. Often when discussing rock music, this becomes a discussion of distortion and syncopation. Again, this is divided by culture, so not always consistent. However, one argument that seems to come up often is that if the music is causing the body to overpower one's thoughts, then it is a problem, and that is most often related to either issues with syncopation, certain types of harmony, or with loud bass frequencies which resonate parts of the body. Ideally, the most fundamental groups would prefer music which has the most natural tones possible, majority consonant chords, and strong, full cadences, with subtle rhythm, gentle harmony, and with a clear, easily singable melody driving the music, all of which act as a better foil to clearly lift up the didactic message being delivered in the lyrics.
Finally, there is the concern about the musicians themselves. This is generally not as much of a concern in the most liberal or contemporary groups, but it can be a large concern for more traditional groups. The consideration here is that no matter how good the song, there is some influence by the performer, if for no other reason than association or the implied condoning of associated behavior. This was a big concern some years back with Amy Grant because of, among other things, her divorce and remairage. For more fundamental groups it was considered that listening and supporting her music was similar to condoning her immoral actions to younger Christians. As is clear from record sales, these same factors are not considered by more liberal and more modern cultures.
How this applies to Rock music itself
Lyrically, Rock music can and does easily extend all the way from dark, overtly satanic into very wholesome lyrics. While Rock is known for "Sex, drugs, and Rock and Roll", the genre is far from limited. There are some bands which lyrically are not explicitly immoral. Therefore, it would seem that the conversation is forced out to the other considerations if we were to condemn it generally.
Historically, Rock music rose to prominence amid a culture that was blatantly immoral and was used to spread and popularize that immorality. For that reason, even when other elements are not present, there is at least the implication, at least amid some cultures, especially older or more traditional ones, that condoning Rock music is condoning the immorality of those cultures, or the general secularization of the current one. I believe that this is the greatest influence on held convictions. Even those who do not care about the association of individuals would likely still understand the broader cultural relevance and weigh that in the consideration.
For those who do consider the music itself, the common use of heavy distortion, along with heavy syncopation, strong, unresolved consonance, and deep bass and rhythm which tend to override the senses, compel one towards anxious action, and which muddy the clear melody, make Rock music one of the worst offenders. Nevertheless, this is only generally so, and there are many exceptions. However, unless you filled your collection selectively, you would likely find most of the music falling across the line of offense for somebody.
As I have stated, I do believe that much of the dividing line for this conversation is between cultures and not specifically denominations. In general, age will play a factor, though a denomination's teachings about Biblical separation will also play a significant role. It is even quite common to see churches which cater to both crowds by having a separate service for each while still being the same congregation.
However, some denominations will be particularly opposed to Rock music regardless of the age and other culture of the individual. For instance, any Fundamentalist denomination will likely oppose it as they believe in a strong degree of separation from secular culture and have ties to more traditional cultures, and they would rather err on the side of caution. There are also denominations which may oppose music entirely, or perhaps instruments. Those would be for different reasons.