The basic gameplay model you are going for - switching between different characters with different skills to solve tasks - has been done before, but it's still fun, and done before is pretty much what the Iphone is about, right? It looks good, some of the gameplay elements will work brilliantly. I liked flying to get over holes and not slip on ice and being smart enough to read. As long as you can make all the characters having different skills make sense by giving all uses, and introducing a balanced number of stuff each of them can do, that'll be a big selling point.
Got some questions and feedback, though:
There was something about finding treasure. Is that going to be just for points or getting through the level, or are you introducing some RPG elements?
Will the dwarf have (as) many special abilities as the gnome and fairy, or will he just be a better fighter? I think it can be fun if you want to mix RPG and puzzle, but if you make one character better at puzzle, and one better at RPG-fighting, you're better off sticking to one character. I think the best option is for each of the characters to have strengths and weaknesses both in getting around (gaps/ice) and in fighting (different kinds of enemies). Or are you approaching this from another perspective?
Are you going to include a map or a mini-map feature? If the places the player explores are as big it looks like from the trailer, that'll be very helpful to avoid backtracking and confusion.
About that - avoid backtracking, or make it fun. And fun does not mean going through the same fights or puzzles again, it means doing something differently. A puzzle or timing puzzle that you have to do again, but backwards, thus making it a new experience, is pretty cool though.
I'm not familiar with the Iphone as a gaming platform, but is this game going to be completely based on touch screen? That might make it easy to get into, and with intuitive controls, probably a great game for a phone, but fighting might get a bit tedious if it consists of clicking on the enemy once and waiting until it goes down. Not a game breaker, but how have you planned the game controls?
That's all. It looks good, so please show more. I don't have an Iphone, so if something I've asked would be self-evident if I had one, please say so.