I'll say that in the typical fashion of the recent 3D Sonic games with Red Star Rings, there are about two or three that are OBNOXIOUSLY difficult to get. Not because they are hard to find, they are just really difficult to obtain once you find them.
In general though, finding the Red Star Rings is more fun than in previous games, because they are well hidden, and the level design really is built for exploration. You'll need to master the parkour moves to find them all though, some of them are hidden around walls and what not (usually hinted at by a trail of Rings).
As for animals, they are also fun to search for. But they are infinite, meaning they are not exactly the same kind of collectible as Red Star Rings, they are more like Rings. You need them to progress through the story, as the last level in each world requires a certain amount to progress. Though if you ever find yourself short of animals, then to put it bluntly, you're not very good at the game at all! Seriously, the animal quotas are so easy to meet. The highest quota is 7,000 animals. I had a lot more than 10,000 when I reached the final world.
EDIT: So I know I have hardly even mentioned the story/writing in this game.
Well, here's some tumblr posts by another person that I fully agree with, regarding the portrayals of Sonic and Tails. Seriously, the plot of the game may be simple, but the character interactions and how they are written are arguably the deepest and most well written they've ever been. And the humour is bloody brilliant, incredibly sharp, and in general, this game makes excellent use of body language. There are spoilers a plenty in these posts, but if you don't mind that, give these a read:
http://darkchaos-chao.tumblr.c...character-sonic
http://darkchaos-chao.tumblr.c...character-tails
I'm now extremely maddened that reviewers like IGN brushed this game's story off as a "Saturday Morning Cartoon", or that it over uses the word "bro" (IT'S RARELY USED. Sonic simply calls Tails "bro" on occasion, which is PERFECTLY in character for him, especially considering how their relationship is explored in this game). While the cutscenes may only tally in at about 30 minutes, which is about the length of your average cartoon episode, they explore SO many deeper themes, and the voice acting is so well done, that brushing this off as a "Saturday Morning Cartoon" is just offensive, and incredibly disrespectful of the effort that was put into bringing some genuine emotions out of this game's plot.