Okay, given my day off from work I've decided to hunker down and get back to where I was. I've currently made it to Cleansing Waters, but my notes are so long at this point that it's better to post what I've got so far.
BUGS/GLITCHES-The properties of Shield seem inconsistent. Against projectiles, it works as expected, but some walking enemies (like the giant bird) seem completely unhindered by it, walking through it and damaging you (duck-shielding on top of the pouncing cat also causes it to glitch out), while others (like the fast plant enemies in The Lion's Den) die upon touching it. I realize having them be deflected might cause some collision issues, but some consistency would see me using it a lot more.
-It is impossible to pause or quit the level Let's Run!, even if you've already beaten it before
-On occasion, selecting "Continue" will start you directly into your last level, rather than the map screen - a real problem when you have no abilities in an impromptu boss fight.
-On occasion, entering/exiting the Guutara boss fight will cause the game to Save (play the Save sound effect), regardless of whether you beat him or die.
-The water sounds in the level Cleansing Waters is a bit too loud and repetitive, and seems to glitch out after the first screen (unless it is purposely a different effect).
FEEDBACK-Touching the root of the first Plant miniboss as she dies still hurts/kills you, even though the head and stalk do not.
-In my opinion, touching the head of the Plant miniboss should not harm you. Given that it's constantly moving about and her only weak point, it's enough of an ordeal to time your jumps in between her attacks without having to pray that her randomly-bobbing head doesn't happen to move forward when you attack. Sure, her stem should still hurt, so that Jack can't just run behind her and cheese, but it seems avoiding her projectiles should be the challenge here, not her face.
-The spitting purple flowers could stand to pause a bit longer between shots; getting in close is a good test of one's Dodge reflexes, but there's not enough time in between shots for Jack to dodge and get an attack in without taking a hit. Considering that Jack can't simply avoid them, it seems the only non-damaging tactic is to snipe them from afar. It's the best way, sure, but it shouldn't be the only way.
-The ability souls could stand to stay around a bit longer, say, half a minute. I recall a focus of the game being swapping out souls on the fly to deal with unique encounters, but when souls vanish in all of ten seconds, too often I find myself stuck with a bad ability I only intended to use for a moment.
-I'm sure a gem counter is in the works, but it wouldn't hurt to also explain what they do. For the first set of levels I was under the impression every hundred or so gave me an extra life (in addition to the full heal), but when received a different "1UP" message I realized it took way more to earn an extra life. "NICE" doesn't really explain much.
-Please require two presses of the Escape key to quit, and add a prompt "Press Esc again to quit, Q to Pause" after the first press. This is an issue in a lot of Game Maker games, but it's most aggravating here.
-Please make pressing the Q key un-pause.
-A jumping buffer after falling from a ledge would be as welcome as ever; the frustration of the game's single-tile platforming bits stems from the jump command not registering because Jack's already slipped off the ledge (probably because Jack's collision is narrower than his sprite) and dropped like a rock. This is likely the reason why people despise the Speed ability, as it shortens the timing window even further, and makes the platforming feel very bare-bones and unpolished.
-The Gas Power, in general, seems like a bad choice in most situations. Unlike the dichotomy between Boomerang and Bullet (Bullet has longer range, but doesn't stun), Gas has pathetic range and fire rate, and is usually only dropped by Plant enemies, who are immune to it - meaning it's useless against a large portion of the enemies in the levels it appears. Gee, it'd sure be cool to use Gas's unique penetration against that mushroom enemy garrisoned at the entrance of The Lion's Den, but too bad, he's immune.
-It seems strange that the crushers in Seismic Activity would hurt you when touched from the side. Sure, the Thwomps in Mario hurt when touched anywhere, but that's because they were clearly covered in spikes; by contrast, the wooden crushers in the dungeon levels were harmless from the side, letting the player run up against it to get revved up. I'm fine with having them hurt, but a visual redesign would go a long way. Glowing lava pillars, maybe.
-A pet peeve, but enemy projectiles don't seem to be bound by the same rules as Jack's. The flying enemies fire tracking Bullet shots that go right through the crushers, whereas Jack's Bullet shots can't. For such a rich environment, it really ruins the atmosphere when established physical laws are voided for the sole purpose of screwing Jack over.
-Kamikaze enemies probably shouldn't drop coins/gems unless killed before they commit suicide, thus encouraging players to avoid them or take them out beforehand.
-It'd be nice to have a shop earlier in the game, selling only the Hint and Save functions.
-Several of the enemies could use 'tells' for when they are about to attack:
- The Boomerang soldier, for when he's about to throw; too many times, I think I have him stun-locked with my attack, only for him to instantly throw out a boomerang and kill me. You already have one made (the cutscene when the wounded soldier is about to attack Jack), and it could be only a split second. But give some kind of prior warning.
- The red Gas-burping enemy, first seen in The Lion's Den. This guy's firing pattern is completely random, has no animation, and does not interrupt his movement. Since you have to jump up and be exposed to progress, avoiding his shots/body on the narrow platform is purely a matter of luck.
I'll post more as I get farther.