@Landon: It's even more upbeat and colorful than anything else Shin Megami Tensei. I might wrong, but something about anime characters dancing around to playful music certainly seems like it would appeal to a larger audience than silhouettes and typography. Then again this all based on my assumption that this stuff appeals to the broader anime fans at all.
@Zeik: That's why I said it was more the cut scenes (ski/beach trip, hot springs) that were about broadening the audience. Especially since those are the things that are easiest things to advertise to people who aren't already fans of the series. They definitely made P3 a game a lot more people could play but I think they're using P4's story and characters to further achieve that mass appeal. I guess it's more that P3 was successfully made with a broader audience in mind and P4 is just further iteration on that idea. Sort of like with Elder Scrolls.
I would say Persona 4 aims even broader and that seems to be successful. Although I haven't played it, all the new cutscenes (like the intro) in P4 Golden seem to aim for an even broader audience than the original.
You could also argue that League of Legends has streamlined a lot when compared to its DOTA roots and that thing is one of the biggest games in the world right now.
I had to restart the on-rails turret chapter of Metro 2033 multiple times because I would hit checkpoints with only a couple hits left. I eventually turned the difficulty all the way down because I just didn't want to deal with it anymore. Is the slime corridor after that? Because I don't remember having any problems with the game past that point.