Actually, the fun part is that if you think it's just an improved PS well... It's a significant bit closer to OfA than you'd think but most people don't check. Heck, I'd argue it's also got aspects of im@s PS3 in there too. Or some SP.
Let's say this. There's actual major plot points. One of them (it's the spoiler thread in question) most people haven't seen yet. You might suspect PART of it, but one part WILL catch you by the curve.
There's also a significant running storyline, but most people will miss that unless you try out the various non live options. It's not that far off the track but... if you focus on levelling your idol above all else, you won't see it for a long time.
There's also a STUPID amount of content interdispersed. If you can name it from im@s there's actually decent odds you can find it there, as a unlockable, unless the song is in catalog 1, or was already part of the PS DLC set.
I'll start off with one surprise - The original THE iDOLM@STER is in Stella Stage. So is 2nd Mix.
Weird when you consider that the original was retired FOR 2nd because of the move to second vision.
There's 35 trio/Quintet, and 2 medleys that are NOT DLC, and I suspect there's still some left I haven't found, and I've got ~70% of the game EX'd. The hard part is 'finding the live that has it as an unlock' although the CP grid has some as unlocks.
It's basically a spin on OfA but you have to explore the menus and do more than the 'Run a live, clear, run to next live' in the solo chain (even though that's actually the most inefficient way to play due to how Extreme Clears double all rewards gotten) because some events will only unlock if you EX clear events in the chain (as well as various unlockables)
It's the sort of slow play, but there IS an end in sight, and unlike PS, it's all dependant on you and your progress in the game. I've no idea how many items are in the game, cause I haven't done a count yet. Just say 'A lot' for the time being.