Actually I'm kind of surprised by that. I could easily see how the lyrics could be a turn-off (depressing, uninspiring, cynical, anti-religion/christianity, etc.) but for me it's hard to see how the music is bad. Oh well, to each his own.
While that's true, I don't think you can separate the music from the lyrics. Dream Theater have a good number of songs that deal with depression and other negative emotions (like "War Inside My Head" and "Honor Thy Father") but they also have a number of uplifting songs. Sometimes, their songs take you through a range of emotions, like how "The Glass Prison" begins with the lyrics expressing hopelessness but concludes by giving the listener a sense of a fresh-start; or how "Learning to Live" takes the listener on a journey from self-doubt to renewed confidence and comfort with the world. With those bands you mentioned (the ones Bosk doesn't like), that almost never happens.
For example, compare "Six Degrees of Inner Turbulence" with
Fear of a Blank Planet. Both are about the sort-of "first world problems" people go through in modern society. While I like both, FoaBP is pretty negative and cynical from beginning to end. There's never really a moment of respite from all the negativity; just varying degrees of "life sucks". With SDOIT, there's also a lot of that, but there's an equal amount of, "you know, we all have to deal with these kinds of problems, but sometimes it isn't so bad and most of us have people who love us helping us along the way". I think that is ultimately why I like DT better than PT or Opeth or Pain of Salvation. DT, from the beginning, have always been about taking the listener on a journey through a range of emotions. Those other bands get way too mired in melancholia sometimes. If Dream Theater are the Charles Dickens of the progressive world than Opeth and PT are like Franz Kafka and Fyodor Dostoyevsky; good, but you can't read shit like that all the time.
/literature snob
EDIT: Also, I'm thinking Bosk should do a song roulette.