Umihara Kawase is a fantastic game. Problem is, it's also unkind to new players, and one that very few will stick with to the point of mastery. The mechanics are more intuitive than they may seem, but the game does a poor job of teaching them. You will only succeed with lots of study or trial-and-error, and most players get turned off and pick a different game by then.
I cannot stress how important a good first level is. They can make or break a game. Many players are very impatient and have plenty of options, so you better make it count. Some of the most famous designers understand this.
In Super Mario Bros. 1, you start on the left end of the screen, facing right, with a bunch of negative space. This implies that you should go right. You're given a chance to see how the mushroom moves before you get it, and how it changes gameplay. The different pipe elevations teach you that holding A longer makes your jump higher (with no text) and being trapped in the pipes but being safe means you can try getting a running start and experiment with the game's physics. The hidden 1-up is placed in a part where many players will discover it by accident. Going in pipes and getting lots of coins tells you that you should try going in pipes. And this is all done without words.
The first stage of Mega Man X1 isn't just a good stage, but an excellent tutorial for new players who aren't that familiar with the conventions of video games. Subtle things like Mega Man X shooting on the main menu (which implies charging and shooting is possible), shots going through the legs of the slow-moving tall enemies (teaching you to jump and shoot), sliding down the wall (teaching you that wall sliding/wall jumping is possible), and suggesting that the player needs to find armor upgrades (when Zero shows up, easily defeats Vile, and he has armor while MMX does not).
Similarly, Sonic 1 turns what would be a boring tutorial into a setpiece. Just by watching him go, you are being fed valuable information about how the game works. If Sonic goes fast enough, it's not just cool, it's USEFUL. He can go through loops, smash through walls, fly off ramps, and becomes more powerful against enemies. This encourages you to go fast rather than play it cautious and safe.
Umihara Kawase, on the other hand? I have showed the game to plenty of players. They play it once, struggle to understand it, and say "that was cool, thanks for showing me" before leaving the game for good.
First off, the reaction to seeing the lesson videos is "Who's doing this"? After you press Start, you should be playing the game right? Players don't want to watch videos, they want to play the game and figure it out. And while I mentioned watching Sonic go as a positive, that's something the player figures out and earns before learning a bunch of things about the usefulness of going fast and why they gotta do it. The one lesson you get in Umihara Kawase doesn't cut it.
The first thing players struggle with is catching fish. They hit the fish once, which makes it dizzy, then they run into it and don't know what to do next. It's not obvious to anyone that hasn't read the manual or watched tutorials that you need to hook the fish AND run into it to catch and eliminate it. They also struggle with urgency- a respawning enemy will almost always get them before they figured out where to go next.
Another thing is one of the game's most useful techniques- the Backsling. Players need to go up, but how? Almost every player thinks they need to press up to take them upwards, but they actually need to press down while holding the hook button. This isn't self-explanatory for most players. How will pressing down help them upwards? Halfway through the first level, Field 0, and the game has already thinned the potential player base quite a bit. This is bad, and this is probably how it's always been.
How would I fix this? Here's an idea- on Field 18, there's a conveyor belt that pushes an extra life off the field, and you can get it if you're fast enough. What if something like that were in the first level? The conveyor would only be on top of the surface, and not on the wall next to it. There would be a pit underneath. You would attach yourself to the wall, then fling yourself on to the conveyor after the extra life falls and you press down to try to get it, missing the chance but being flung upwards. That's how a player might be taught that pressing down flings you up- and next time, if they do it fast enough, they'll have enough time to get the extra life.
For enemies, you have an enemy that's either trapped in a shallow pit (meaning you can still hit it but it turns around) or on a small platform (meaning it can't get to you) and you can get it from a safe distance, catch it, and have a chance to see that enemies respawn before you continue, being given a decent chance to run away fast enough.
These are good design ideas to subtly teach new players about the game, and neither of them involve taking control away from players with video lessons, text, or having to read manuals. The rest of the game is a masterpiece that deserves to be played by more people, and a better first impression could have sold it.
How would you have designed the game better?
Any game with a learning curve as sharp as Umihara Kawase will throw off 99% of players before the first Game Over screen. There are any number of little tweaks possible to address this, but I'm not sure I'd want to implement any of them. It's ok for a game to be niche.
But to answer your question: speaking for myself, the number one gameplay factor that nearly threw me off the games altogether was the random enemy spawn. It leads to endless bullshit, unavoidable deaths during the first few dozen hours, until you develop the sixth sense required to predict when and where the random thing is going to happen. If they wanted to randomize enemy layouts, have six possible spawn points per level and have the game randomly pick four spawns when the level starts, or some variation on this. No real-time spawning, though - it just messes with the flow of gameplay, and provides little benefit. I guess it was included to dissuade loitering but the level timer would've been fine for that.