This is pretty fun and challenging and I like how you broke it down to small levels. Your previous game was a bit cumbersome, especially with the long in-between levels where if you screwed up, you had to start all over, again and again (almost rage-quit level of frustration). This one is nice being able to tackle levels in any order.
The ice levels took some getting used to. A tip, if you let go of the movement while jumping, you can land without sliding. One level (forget which) requires using a left-right 'glitch' to pass some spikes. Basically hold right for a few seconds and press left, then you can jump at full speed immediately. Only had to use that once, though.
Mine-cart levels were a bit annoying at first, but you can memorize the timing, more or less. Secret mine-cart levels were more random with the spinning spikes, so those may require a lot more repeats to get through. If you're close to the end and drop, as long as you pass the edge of the screen, it should pass that level.
Few levels that might be broken, Secret 31, in the middle where the trophy is, I can't find a way off that platform due to the spacing of the blocks up to the sides. Running off lands you in the fire pit and jumping, you always hit the block. Unless there is special technique, I don't see how to progress from this point. Can't get off that platform either way.
Area E, level 5, I can't find any way to the trophy. Unless it requires absolute perfect timing with the doors and the entry is right below, I can't find a way in.
Area D, level 3, as mentioned, can't find the trophy anywhere.
Kind of waiting to get those last three before moving onto the boss level.
Also seems to be a problem with Flash crashing if the game has been running a while. Not a big issue since you can just reload and the levels are short enough that you don't lose much , if any, progress.
Overall, a great, challenging game. Some levels are a cakewalk, others quite challenging, but overall nicely balanced.