I have never been much of a fan of horizontal shmups (vertical shmups are the best game type in the world
) but you seem to have captured the essence of shmups in general. You can upgrade your weapon, there is enemy fire all over the place, and you can kill tons of enemies. The game is good for what it is
Now for the critiques.
It is easy to keep dying immediately after you revive. This could happen because the revive point is inside of a wall/enemy or you revive right on top of enemy fire. Most games (shmups included) resolve this by making you invincible for 1-3 seconds after you revive. You could also have specific revive points which you know are safe, and have the player revive at the last one he passed. A revive point could be obvious with a graphic and everything, or passive and the player has no idea he just passed it.
It is also easy to mistake enemy fire for an upgrade, and vice versa. Perhaps contain the upgrade in a bubble or some other easily identifiable object. The problem with trying to find a solution is that you need to be equally aware of both upgrades
and bullets.
There should be some visual and/or audible clue that you have collected an upgrade. Also, when you upgrade to the next power level (2.0, 3.0, etc) it would be cool if your ship changed slightly.
The credits system gives you the feeling of being in an arcade. However, it is a little annoying to insert all 3 credits each time. Maybe have a button which automatically does that and starts the game? Alternatively, you could have something in-game change depending on how many credits you enter at the start... After thinking about this, inserting credits is how you choose difficulty level. Maybe make this a bit more obvious, perhaps with a menu.
A pause feature would be nice. The "P" key, return, or the space bar would work well for this.
It took me about an hour, but I finally beat the game
Although it took forever, I had fun playing.