Guitar Hero 5 -- that's the next full-band installment in the franchise, if you'll remember -- keep on piling up. The latest addition is what Neversoft calls "challenges." Sure, the name lacks creativity, but the feature itself is ingenious -- each song contains an additional goal that, when completed, unlocks new items for your faux-rocker.
For instance, the end of David Bowie's "Fame" contains the titular word 24 times in a row, starting at an inhumanly high pitch and then descendng. If you hit five of the "fames," you unlock the gold challenge record, 20 "fames" nets you a platinum challenge record, and hitting all 24 "fames" gives you the diamond challenge record. Also, if you can make it through all of Rammstein's "Du Hast" without tearing your ears off, you get a multi-diamond challenge record.
Okay, that last one was fake, but for more real song-specific challenges, Destructoid's got a huge list of examples.