But back on subject:
While the absolute quality of a band is difficult to judge objectively, it is possible to judge a band's strengths and weaknesses relatively objectively.
For example, if I were to claim that The Black Keys are particularly strong at writing complex and innovative songs with superior advanced musicianship, I'd clearly be full of shit. Their stuff is damned catchy, and I enjoy it immensely, but superior musicians they ain't. They can write a hell of a hook, and they have a lot of energy, which are their strengths. But sheer musicianship, no, they're not strong musicians.
Some music really has nothing going for it. It may be perfectly decent in execution, but is uninspired. While it may inspire emotion in some segment of the audience, there's not a lot of inspiration behind the actual music. This kind of uninspired musicianship (which is what a lot of mainstream music really is nowadays) results in poor music.
Some music has some things going for it and lacks other things. This is most bands....there is some bad and some good, and if you like it, you'll focus on the good parts, and if you don't, you'll focus on the bad stuff.
A small quantity of music is just fucking brilliant and pretty much lacks anything bad at all. This is normally restricted to individual songs and occasionally extended to albums. People who dislike this stuff mainly give reasons like "I don't like that style" or else just lack any semblance of taste. A good example would be Source Tags and Codes, by Trail of Dead...there is no legitimate reason to dislike this album with the exception of taste. You could also argue that Funeral, by Arcade Fire, fits the same bill.
These are not arbitrary or subjective. They are very real differentiations one can make from a relatively unbiased perspective. Where, to use a previously mentioned example, would anyone place American Idol stuff? I think it pretty obviously falls into category 1. It's uninspired, it's not really even art. It's performance. Category 2 and 3 (and quality within category 2) are determined to a large extent based not on what the band is trying to do with their music, but rather on how well they execute a particular objective. There are plenty of songs that attempt to convey a particular idea or make a particular sound that either seem forced or bungled. To deny this fact would be naive. It doesn't matter if you like what is being attempted, it matters how well that attempt succeeds, and as far as that goes, it can be considered from a much more objective perspective.
