The best example of an album to test with is; Pink Floyd - The Wall.
If you are familiar with the album, the lack of gapless playback will drive one insane. The entire 2 disk set has NO gaps between songs. The end of each song carries over to the next song.
Now, what's weird is in Windows 8.1 if you create a playlist of the entire album, and play the playlist in the old Windows Media Player in W8, it plays fine with no gaps. But, go with the Modern Music app, and play that same playlist, you get gaps.
And of course, in WP8 Music+Videos app, same playlist, GAPS.
AAAAaaaaaaaaaaarrrrrrrrrrrgggggggggggggghhhhhhhhh!!!!!!!!!!!!!!!!!!!!!!
(it's one of my favorite albums)