JoeinVegas raised a really great point, that sometimes we not only like songs but like specific bits of songs.
Hmm... Well, I'm a sucker for any laughter on tracks (generally unintentional), so I would heartily agree with the Janis Joplin chuckle. I would also proffer the more bittersweet laugh in "Fighting Talk" by Everything But The Girl; and a track from the third Lloyd Cole album, Mainstream (I think "Sean Penn Blues") that also features a chuckle inbetween the verses.
For other classic bits you have to have that wonderful feedback sound at the start of "I Feel Fine" by The Beatles. And I know I keep harping on, but the final harmony at the end of the Beeb version of "To Know Her is To Love Her" by The Beatles always melts my heart.
I also have a profound love for the live version of "Common People" by Pulp that was recorded at Glastonbury '95 (on the Mishapes version of the Mishapes/Sorted for E's and Whizz single): not least because of the way Jarvis exhorts the band to "take it down" - first saying it quietly, and then, when they clearly haven't obeyed, he makes it a real instruction. Oooh, makes me go all dithery!
What else? Anything where strings sweep in, especially in pop. That definitely means the entry of the cello to Michael Andrews' version of "Mad World" from the Donnie Darko soundtrack. Gratuitous use of strings? I'm there for it.
Crescendos also get me in the zone of joy: there's a crackingly prog-rock bit in Orbital's "The Box" (1996 single - I think it's the third section) where it goes very quiet and tinkly - hey, check out my musicological language! - and then very loud, almost like a church organ crashing in. Oh, and talking of organs and crescendos, GodSpeedYouBlackEmperor are the Gods of crescendo.
Better get off. I may try and link these but Cloud calls me for my tea!