I agree, great idea for a thread.
Nostalgia, eh? You're asking the right person there, young lady!

I pesonally find that there were songs I would not listen to --- essentially would refuse to listen to --- when I was younger and more cocky, thinking I knew everything, like METAL ROOLZ! But as I grow older (what's that you say? Not possible for me to grow any older? Cheek...) I start to appreciate them more not for who the band are and whether I can like them or not, but for what the songs are.
ABBA are a perfect example. I used to jeer anyone who liked them, which was obviously both a sense of insecurity and a result of peer pressure. Without that, I can see the band were outstanding and had many great songs. When you're younger I think to some degree you decide what is cool and what is not, and even if you hear a track by a band not considered by you (or your mates) to be cool, you decide you don't like it. Now I tend to enjoy music for what it is, not what I think, or others think, I should like, hence my often boring and pedestrian music tastes. But that's ok: I'm never going to be into, for instance, Death Grips, no matter how many people tell me they're great. Just not my kind of music. But now I know that: it's not just a case of feelign like you have to like/not like it.
Memories can definitely be stirred by nostalgic songs, with comments like "Oh I haven't heard this for ages!" but generally I find that by and large the music I listened to as a youth is still in my collection today. There's nothing really that I don't listen to any more. I've just expanded my tastes a little. However, unlike Fluff, I wouldn't listen to some crap just to be reminded of better times: if I hated it then, I hate it now, and by hate I mean genuinely made that decision not had it made for me as above.
Christ, I do ramble don't I? Where's my cup of milky tea? Must nearly be time for my afternoon nap ....