Trophies were a great addition to the Smash series. It makes SSB not only a great game with some of the best Nintendo characters pitted against eachother but a virtual book on the history of several parts of Nintendo's history. It has more of an effect if your new to a majority of Nintendo's franchises. For example, be honest how many Nintendo franchises besides Mario, Legend of Zelda, and Pokemon would you have known about had they never included Trophies. Though I did notice Brawl didn't really give enough for franchises like Earthbound (I mean even the 3rd Party characters (As much as I like Sonic) got more trophies then them. Earthbound fans have the right to be mad about that), I sure hope SSB4 has great representation of all the franchises covered.
Stickers aren't really that useful. They seem only to be there to include more stuff without having to make tons more Trophies since the artwork has already been done for a Sticker. Problem is without a proper description like the Trophies do, some people even some experienced gamers may have no idea what a few of them are.
CD's were cool though collecting them was quite annoying until you made the CD factory thing (Which sometimes still takes hour upon hours to do). I just realized, I wonder how huge the soundtrack for SSB4 will be. I'm going to assume that SSB4's content will put Brawl's to shame just as Brawl did to Melee.