This is supposed to be a light discussion, but I'm one of those guys who is seriously affected by music. I love good music, but bad music is like poison.
To me good music is music that has a good reason for being written. If there's no good reason why you needed a new song or a new symphony, it shouldn't have been written. This includes anything that sounds like something else that's already been done (90% of what Boston ever recorded, plus a lot of bebop jazz tunes, plus a lot of symphony music).
Another category of "music" that shouldn't be recorded is the stuff that falls into the category of pure garbage. This is a large category which includes rap, hip hop, country, most bluegrass, and most commercial pop offerings since the mid 80s. Also a lot of orchestra music is crap, for instance Stravinsky's abstract stuff that sounds like a movie without the movie.
You know you have good music when you can't help whistling the tune later, or it has a great interplay between different instruments, or some other really catchy hook. The folks who know how to write good music have my everlasting respect. It's great stuff, and I always thought it should be obvious why it's great. For instance, the Beatles. It's been over 40 years, so maybe some of you don't know. If you want to know what real songwriting is, check out the Beatles. I grew up listening to rock and roll and I love it, but I'm not stuck in one era. I love Benny Goodman and Glenn Miller, etc. I love the classic blues artists, and a lot of the modern ones, and the great symphony composers such as Vivaldi, Tchaikovsky, Beethoven, Mozart, etc. I love jazz, but I don't care for the guys who were great technicians with no soul. That stuff just wears me out. I'm a sax player, but I can't listen to Charlie Parker. Could you try not cramming so many notes into a song for crying out loud? Charlie Parker should have calmed down and listened to Paul Desmond play the alto sax. Or if you like fast notes, how about Gerry Mulligan? Both of these guys were so much more musical than Parker.
Some modern stuff is great, too. I like Adele a lot, and here's a great way to use modern technology to make inspired music:
http://www.youtube.com/watch?v=z0PGK7a2IFo I can't help noticing, however, that the modern era is sorely lacking in genuine high quality music. Even the pretty-good new music seems to fall short in lack of development within a song, poor balance between instruments, boring melodies, etc. The bad stuff is just unbelievable. Miley Cyrus? Lady Gaga? What the hell is that supposed to be? Last year our blues band was loading out after a bar gig, and they fired up the jukebox and started dancing to Poker Face. Holy Crap! How did somebody allow that to be recorded?
This is somewhat of a mystery to me because during the classic rock era, with megacorporations firmly in control of the industry, a lot of really great music was recorded. Today, with the same megacorporations in charge, they seem to have a policy of releasing 95% garbage to the market. I'm not sure I understand why...
I have two teenage sons. I always expose them to great, classic music. We play in a swing band. They love rock and roll, jazz, blues, orchestra music, and a lot of things that don't fit into a category, and I feel pretty good about the fact that they can identify exactly why some music is good and some music sadly fails to measure up. For example, although I generally prefer musicians who learn to play real instruments, there's nothing wrong with techno music, but some of it sucks for various reasons. Check out Erasure, a techno band from the 80s, and compare their lackluster effort to Depeche Mode, another techno band from the 80s, particularly their Violator and Music for the Masses albums. Depeche Mode's songwriting, layered arrangements and inspired producing are extremely impressive.
I'm sure somebody out there is wondering how I can dismiss the entire category of country music so easily. I have a reason for that. The American musical tradition gave us ragtime, jazz, blues, and rock and roll. That's an amazing heritage, if you stop and think about it. These forms are so creative and full of surprises. How did this culture also give birth to the disabled child known as country music? In country music there are no surprises. The chords are the same, the stories are the same, the voices are the same, the instruments are the same. When you hear the beginning of a country song you know exactly where it will end up and exactly where it will go before it gets there. And don't get me started on rap, hip hop and whatever they're calling the mess that heavy metal has degenerated into (hardcore, deathcore, screamo.... even the names are a bad joke.) I'd like to go back in time to listen to Benny Goodman and his swing orchestra, then meet them backstage and play some screamo for the boys. What a laugh.
rock, hard rock, heavy metal, etc. Some classis rock here and there. 
