Alright, I saw enough feedback to decide to give this game a go, and enjoyed it quite a bit.
First world went fairly well, although there were some parts where I experienced some lag. Also, in that Snowball segment, I think it would look a bit better if there was only one giant snowball chasing you instead of having three spawn. The first boss was a bit tricky for being the first boss you face. That random icicle burst at the end of the phase was probably the trickiest thing to dodge. World 1 end boss was bit tricky, but it's a pretty fair boss, except on those occasions where the yellow energy attack would spawn in an impossible to shoot without dieing place, but you can for the most part get the shots in on it if you position yourself well.
World 2 is where things really start getting interesting, I liked the fans, although it wasn't super obvious at first which fan does what and it can be a bit hard to see at times. I don't know if it would be possible to distinguish the different flows of air, such as by colour, but if not, I think people should be able to handle it. The W2 miniboss has very few phases, although they can be tricky to learn at first, it only becomes natural once you've figured out how to just move your way back and forth staying away from the projectiles.
The lava world part of W2 was definitely a very fun part though. This is where you really start to introduce some very fun tricks. The firewheels were executed very nicely with some neat segments requiring you to reverse the direction. And the lava immunity bubbles were great, and it changes the water type platforming since you have to protect the bubble as well as the kid.
The World 2 End Boss, I liked it. The only thing I had a problem with was that random charge he does at you after the 2 waves of boulders fall. There's no real way to know it's coming and you have to be prepared to fall at just the right time and it just seems to be really annoying. Otherwise, I'd say the boss is one of the best in the game.
World 3 is the best in the game. The space segment was Excellent®, and you did a great job making sure people don't get screwed over by not knowing where to go in anticipation of attacks. Staying in the laser guided area was pretty dang precise though, I probably would have had some more issues with it, but you kept the diagonal segment short enough that I can live with it. The Space Monster Boss, I liked it, not so much the getting hit from behind part but the attacks were tricky yet dodgeable. Very well done.
The laser jumping was fun, hard to make at times, but still fun.
But the part I really liked was the physic changing areas. Green had infinite jump, Yellow super speed, and Red Extra Gravity. All those puzzles were very fun and unique, especially the ones where the area switched between all 3, superb segments.
As for the boss, I unfortunately don't have the time right now to fight her, but from what I've seen so far, she looks good, like the shooting star attack, very cool looking.
The charged gun you put to very good use. You give clear indications of when to use it, and it worked really well. I do feel that the cracked platforms you can break could be a bit more obvious. And on all segments where you need to fire back to back charged shots, you always gave the right amount of time, you feel pressured, but you never feel like you aren't getting enough time to charge your shot. For example, the yellow area in world 3 that required you to make 3 consecutive charged shots while doing the platforming to disable the purple barriers, it worked out beautifully and consistently, and I'm pretty sure that was a longer segment, but it was really fun!
Anyway, I really like where this is going, only other things I would suggest is maybe an options menu, for adjusting things like sound, setting the music so it doesn't repeat with each death, etc. But that can wait, I'm not completely sure how many worlds you plan on having in the game. But keep up the good work, and you'll have a great fangame when you finish it! ^^