Does 1999 sound "trendy" as in "it sounded like everything else out at the time" or more groundbreaking and "damn, I've never heard anything like this?"

I think those can both describe "trendy." So the first Guy album was trendy in that 2nd way, because it was fresh and groundbreaking. Same with Illmatic, Dre 2001, 400 Degreez, Thriller (Billie Jean, Wanna be starting something), Chronic (even more than Doggystyle)...even So far gone. While I'd say that the "sounds like everything else out" title would go to music that was made to imitate those albums.

So from there, it's hard to predict what music will end up being seen as "timeless" or still good in the future. And this also leads to another thing. One album may introduce a certain style, but then another album may come out and improve on it and end up becoming the staple or landmark album that folks enjoy in later decades.

With songwriting, I do think it's a lot different. I have noted a few times that Ballads are a thing of the 80's and 90's, and radio won't even play a Ballad nowadays. And I do think some things come in circles...when a throwback sound first appears, you say "this sounds like it's from the 80's" but six months later, you say "this is what 2012 music sounds like!"

And BTW, I hope the She'kspear shit NEEEEEEEEEEEEEVER comes back!! Ever ever!!! LOL