Here is a bit more detail on some of those points. Still trying not to spoil the game for anyone. (Go play and vote before reading this, if you haven't already. >.> )
- The stars are all or nothing. So you don't make good progress towards unlocking the other characters until you have mastered the difficulty of the game. This makes for a poor engagement curve.
- Small rocks is the only wave in the game with no stars, instead it has more complicated patterns that ask you to look ahead and plan your route. This provides a difficult but rewarding challenge. I think if you take a good look at how your game is using stars you will find they currently take away a lot from the game.
- Wave 4 is hype... the first few times. Unfortunately, the star placement is a distraction (try later in the wave) and the wave gets long and easy after the first few times. Still it offers something that no other part of the game does. (Also, your character choice has some amusing effects on the context of wave 4. Not saying I would do anything about it, just that it is weird how much one character can change the interpretation of the story.)
- Encouraging risky choices: Placing collectables in a line leading up to an obstacle to encourage the player to wait until the last second to change lanes. Splitting the path into two routes one easier one more rewarding and letting the player choose which half they want. Offering power-ups or special paths that offer a huge benefit, but sometimes placing them in hard to get to places. Just to name a few I have seen. The advantage to this is it lets players adjust the difficulty of the game themselves while rewarding them for turning up the difficulty. This makes the same level fun for multiple skill levels.
- Yes, that would help it be more re-playable, but you have to keep in mind the type of experience you are trying to make. It is okay for a game to end. Not all runners need to be endless you know. That said the thing I see in games that have A LOT of replay value is that they don't change the way they interact with me, I change the way I interact with them.
I hope that helps.