From the top of my head...
1. An endless sandbox as seen with One for All, because it's piss easy to unlock S4U things for request work.
2. The fact vocal lessons from the aforementioned game weren't nearly as nauseating as was Idolmaster 2's.
3. Bursts and appeals in general -- as they're strategic element in festivals -- which expertise got mandatory.
4. Having the freedom to do whatever through song customization; it really sort of just says "you" all over it.