On his blog, my friend Geoff S. posits that
The word “dated” doesn’t compute for me, at least not as an indicator of quality. I am pretty good at determining the age of songs, but my favorite old songs aren’t those that have aged the best but those that are the best. Erasure doesn’t have better synths than Spandau Ballet, just better songs.
I'd be curious to know what prompted his reflection. I agree with Geoff that dated functions best as a value-neutral term. This is hardly ever the case--witness this RS takedown of The Cure's "Bloodflowers" that uses dated pejoratively, not pausing to explain why a dated arrangement is undesirable, simply accepting that it is.
Dated, however, makes more sense without this hint of judgment. In a few years, when Autotune begins to sound dated to us, as one imagines it will, the term will be able to work as it should, meaning "a sound easily identifiable as belonging to a certain era." Thus, we might say that The Replacements' "Asking Me Lies" sounds dated, as it bears the hallmarks of late-80's commercial pop-rock: precise instrumentation, lite-funk guitar, flat-sounding percussion.
However, dated does carry this hint of prejudice, and I'd argue it's because we like to think of our favorite music as timeless. This sentiment is implicit in Geoff's comment when he says "my favorite old songs aren’t those that have aged the best but those that are the best." To me, it's a strange conception of music as it exists in time. All music ages, and different kinds of music seem to age differently (though that's far too large a topic for this post).
The same songs even sound different in 2009 than they did in, say, 1989, literally and figuratively. In the literal sense, format differences ensure that songs sound different no matter what. In the case of "Asking Me Lies", the difference is exacerbated by the fact that I'm listening to the 2008 CD remasters on my computer in mp3 form, whereas in 1989, when Don't Tell A Soul was released, most listeners would have heard the song on viynl, cassette, or FM radio. The information my computer processes is literally different from what a record player does, and they provide related but distinct listening experiences.
More importantly, music changes over time because we only make sense of it in relation to other music. Today, Erasure sounds different because there are a whole wealth of new sounds it exists in relation to: we could chart its influence on music post-Erasure, or compare it to drastically different songs (the specifics of each listener and listening are relevant here). And it no longer exists in the present, as songs seem to in the months following their release or as they climb the charts. So I'd argue that this has aged such that the listening experience may be as enjoyable as it was in 1989, but that the experience is necessarily a different one.
Moreover, if it's true that songs simply are or are not good and that this judgment is a timeless one, what explains the periodic revivals that certain genres experience among groups of people all at once? The same songs speak differently to people during different times. This is in fact one of the joys of art, that in rereading a book, I can gauge how I have changed because the text has not (ignoring for the moment the literal types of differences discussed earlier). If every reading or listening experience was the same, if works of art just are, I don't think we'd keep going back to Shakespeare or The Beatles. What makes their art "timeless" is a certain richness that manifests itself differently in different eras, the timeless quality actually a perpetual timeliness.
So in the end, I have to disagree with Geoff--though I hope I haven't mischaracterized what he said--partly because I don't like Erasure or Spandau Ballet.