I don't often hear the songs that are big in the States but not in the UK unless I actively seek them. As a result, I've never heard Crash My Party or Sweater Weather, nor older songs like Breathe by Faith Hill.
I've missed out on Janet's discography. I've missed out on Mariah's... haven't heard Britney's first three LP's from start to finish and haven't heard a darn thing from Christina's debut.
I've missed out on Justin's (Timberlake's) work with the exception of 20/20, haven't listened to Bedtime Stories all the way through and have yet to listen to any of Gaga's old acoustic demo's.
Other than that I think i've pretty much heard everything else new millennium pop has to offer.