the variety of the songlist. With songs like This is Halloween and Sentai Express (that's the theme song from the Japanese version of the Power Rangers, by the way) mixed in with more traditional titles like Party Rock Anthem and Beautiful Liar, I found that there was a song everybody could enjoy.
Just Dance: Greatest Hits only includes two songs from this game, and they aren't very good, subjective as this opinion is (Airplanes and Only Girl). At least, they're far from the better dance tracks in the third title. Everything else in the game is taken from the first two games, and it feels like the variety has disappeared. I got bored dancing to the tracks quickly, as there was nothing particularly interesting about them. Well, except maybe Eye of the Tiger.
And here's the weird thing: almost all of the songs present in this title are available as DLC for Just Dance 3 anyway. So if you already have that, there's really no purpose to getting this unless it becomes really cheap; at that point you can treat it as a big bundle of DLC.
The same problems in many of the Just Dance titles persist here, such as finicky menu navigation and lack of feedback to your moves: if you don't get a good score, it's impossible to know why, and without any 'training' mode, it's hard to get better at particular moves in any way other than just replaying the song over and over again.
If you're so inclined, there are other modes of play beyond just dancing to each song, such as dancing to a medley of different titles, a shuffle mode, or 'Just Sweat', where you keep dancing while earning 'sweat points' and burning calories. There's also a weird mode where you choreograph your own moves, though that adds little to the game.
There's no reason to recommend Just Dance: Greatest Hits. A cursory search for the first two games will probably find them for a fairly low price, and if not, you can just get the third in the series and buy some DLC. It doesn't help that I think these songs are far from the best songs in the Just Dance playlist, and that it feels like this game is just treading old ground with few improvements to be had. Just skip it.